diff --git a/snapcraft/application.py b/snapcraft/application.py
index 8b91b6ac3b..6e61ba77b6 100644
--- a/snapcraft/application.py
+++ b/snapcraft/application.py
@@ -22,7 +22,7 @@
 import pathlib
 import signal
 import sys
-from typing import Any, List
+from typing import Any, List, Sequence
 
 import craft_application.commands as craft_app_commands
 import craft_application.models
@@ -37,7 +37,7 @@
 
 from snapcraft import cli, commands, errors, models, services
 from snapcraft.commands import unimplemented
-from snapcraft.const import SUPPORTED_ARCHS
+from snapcraft.const import SUPPORTED_ARCHS, SnapArch
 from snapcraft.extensions import apply_extensions
 from snapcraft.models import Platform
 from snapcraft.models.project import apply_root_packages
@@ -51,37 +51,37 @@ class SnapcraftBuildPlanner(craft_application.models.BuildPlanner):
     base: str | None
     build_base: str | None = None
     name: str
-    platforms: dict[str, Any]
+    platforms: dict[str, Any] | None = None
     project_type: str | None = pydantic.Field(default=None, alias="type")
 
     @pydantic.validator("platforms")
     @classmethod
     def _validate_all_platforms(cls, platforms: dict[str, Any]) -> dict[str, Any]:
-        """Make sure all provided platforms are tangible and sane."""
+        """Validate and convert platform data to a dict of Platforms."""
         for platform_label in platforms:
-            platform: dict[str, Any] = (
+            platform_data: dict[str, Any] = (
                 platforms[platform_label] if platforms[platform_label] else {}
             )
             error_prefix = f"Error for platform entry '{platform_label}'"
 
             # Make sure the provided platform_set is valid
             try:
-                platform = Platform(**platform).dict()
+                platform = Platform(**platform_data)
             except CraftValidationError as err:
-                # pylint: disable=raise-missing-from
-                raise CraftValidationError(f"{error_prefix}: {str(err)}")
+                raise CraftValidationError(f"{error_prefix}: {str(err)}") from None
 
             # build_on and build_for are validated
             # let's also validate the platform label
-            build_on_one_of = (
-                platform["build_on"] if platform["build_on"] else [platform_label]
-            )
+            if platform.build_on:
+                build_on_one_of: Sequence[SnapArch | str] = platform.build_on
+            else:
+                build_on_one_of = [platform_label]
 
             # If the label maps to a valid architecture and
             # `build-for` is present, then both need to have the same value,
             # otherwise the project is invalid.
-            if platform["build_for"]:
-                build_target = platform["build_for"][0]
+            if platform.build_for:
+                build_target = platform.build_for[0]
                 if platform_label in SUPPORTED_ARCHS and platform_label != build_target:
                     raise CraftValidationError(
                         str(
@@ -90,24 +90,21 @@ def _validate_all_platforms(cls, platforms: dict[str, Any]) -> dict[str, Any]:
                             f"both values must match. {platform_label} != {build_target}"
                         )
                     )
-            else:
-                build_target = platform_label
-
-            # Both build and target architectures must be supported
-            if not any(b_o in SUPPORTED_ARCHS for b_o in build_on_one_of):
+            # if no build-for is present, then the platform label needs to be a valid architecture
+            elif platform_label not in SUPPORTED_ARCHS:
                 raise CraftValidationError(
                     str(
-                        f"{error_prefix}: trying to build snap in one of "
-                        f"{build_on_one_of}, but none of these build architectures is supported. "
-                        f"Supported architectures: {SUPPORTED_ARCHS}"
+                        f"{error_prefix}: platform entry label must correspond to a "
+                        "valid architecture if 'build-for' is not provided."
                     )
                 )
 
-            if build_target not in SUPPORTED_ARCHS:
+            # Both build and target architectures must be supported
+            if not any(b_o in SUPPORTED_ARCHS for b_o in build_on_one_of):
                 raise CraftValidationError(
                     str(
-                        f"{error_prefix}: trying to build snap for target "
-                        f"architecture {build_target}, which is not supported. "
+                        f"{error_prefix}: trying to build snap in one of "
+                        f"{build_on_one_of}, but none of these build architectures are supported. "
                         f"Supported architectures: {SUPPORTED_ARCHS}"
                     )
                 )
@@ -132,9 +129,12 @@ def get_build_plan(self) -> List[BuildInfo]:
 
         base = bases.BaseName("ubuntu", effective_base)
 
+        if self.platforms is None:
+            raise CraftValidationError("Must define at least one platform.")
+
         for platform_entry, platform in self.platforms.items():
-            for build_for in platform.get("build_for") or [platform_entry]:
-                for build_on in platform.get("build_on") or [platform_entry]:
+            for build_for in platform.build_for or [platform_entry]:
+                for build_on in platform.build_on or [platform_entry]:
                     build_infos.append(
                         BuildInfo(
                             platform=platform_entry,
diff --git a/snapcraft/meta/snap_yaml.py b/snapcraft/meta/snap_yaml.py
index 62d28e4ea0..31035667a8 100644
--- a/snapcraft/meta/snap_yaml.py
+++ b/snapcraft/meta/snap_yaml.py
@@ -1,6 +1,6 @@
 # -*- Mode:Python; indent-tabs-mode:nil; tab-width:4 -*-
 #
-# Copyright 2022-2023 Canonical Ltd.
+# Copyright 2022-2024 Canonical Ltd.
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 3 as
diff --git a/snapcraft/models/__init__.py b/snapcraft/models/__init__.py
index ccc155652d..e2b8912206 100644
--- a/snapcraft/models/__init__.py
+++ b/snapcraft/models/__init__.py
@@ -1,6 +1,6 @@
 # -*- Mode:Python; indent-tabs-mode:nil; tab-width:4 -*-
 #
-# Copyright 2022-2023 Canonical Ltd.
+# Copyright 2022-2024 Canonical Ltd.
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 3 as
diff --git a/snapcraft/models/project.py b/snapcraft/models/project.py
index abb41fccba..5d7ac0f1fa 100644
--- a/snapcraft/models/project.py
+++ b/snapcraft/models/project.py
@@ -23,7 +23,6 @@
 from typing import (
     TYPE_CHECKING,
     Any,
-    Callable,
     Dict,
     List,
     Literal,
@@ -445,24 +444,18 @@ def _validate_default_provider(cls, default_provider):
         return default_provider
 
 
-class Platform(pydantic.BaseModel):
+class Platform(models.CraftBaseModel):
     """Snapcraft project platform definition."""
 
     build_on: list[SnapArch] | None = pydantic.Field(min_items=1, unique_items=True)
-    build_for: list[SnapArch] | None = pydantic.Field(min_items=1, unique_items=True)
-
-    class Config:  # pylint: disable=too-few-public-methods
-        """Pydantic model configuration."""
-
-        allow_population_by_field_name = True
-        alias_generator: Callable[[str], str] = lambda s: s.replace(  # noqa: E731
-            "_", "-"
-        )
+    build_for: list[SnapArch] | None = pydantic.Field(
+        min_items=1, max_items=1, unique_items=True
+    )
 
-    @pydantic.validator("build_for", pre=True)
+    @pydantic.validator("build_on", "build_for", pre=True)
     @classmethod
     def _vectorise_build_for(cls, val: str | list[str]) -> list[str]:
-        """Vectorise target architecture if needed."""
+        """Vectorise target architectures if needed."""
         if isinstance(val, str):
             val = [val]
         return val
@@ -470,22 +463,11 @@ def _vectorise_build_for(cls, val: str | list[str]) -> list[str]:
     @pydantic.root_validator(skip_on_failure=True)
     @classmethod
     def _validate_platform_set(cls, values: Mapping[str, Any]) -> Mapping[str, Any]:
-        """Validate the build_on build_for combination."""
-        build_for: list[str] = values["build_for"] if values.get("build_for") else []
-        build_on: list[str] = values["build_on"] if values.get("build_on") else []
+        """If build_for is provided, then build_on must also be.
 
-        # We can only build for 1 arch at the moment
-        if len(build_for) > 1:
-            raise CraftValidationError(
-                str(
-                    f"Trying to build a rock for {build_for} "
-                    "but multiple target architectures are not "
-                    "currently supported. Please specify only 1 value."
-                )
-            )
-
-        # If build_for is provided, then build_on must also be
-        if not build_on and build_for:
+        This aligns with the precedent set by the `architectures` keyword.
+        """
+        if not values.get("build_on") and values.get("build_for"):
             raise CraftValidationError(
                 "'build_for' expects 'build_on' to also be provided."
             )
@@ -522,7 +504,6 @@ class Project(models.Project):
     ]
     grade: Optional[Literal["stable", "devel"]]
     architectures: List[Union[str, Architecture]] | None = None
-    # TODO: set architectures to [get_host_architecture()] if base is core22
     assumes: UniqueStrList = cast(UniqueStrList, [])
     package_repositories: Optional[List[Dict[str, Any]]]
     hooks: Optional[Dict[str, Hook]]
@@ -669,7 +650,7 @@ def _validate_platforms_and_architectures(cls, values):
 
         core24 and newer bases:
          - cannot define architectures
-         - must define platforms
+         - can optionally define platforms
         """
         base = get_effective_base(
             base=values.get("base"),
@@ -678,26 +659,24 @@ def _validate_platforms_and_architectures(cls, values):
             name=values.get("name"),
         )
 
-        if base == "core24":
-            if values.get("architectures"):
-                raise ValueError(
-                    (
-                        "'architectures' keyword is not supported for base %r. "
-                        "Use 'platforms' keyword instead.",
-                        base,
-                    ),
-                )
-            if not values.get("platforms"):
+        if base == "core22":
+            if values.get("platforms"):
                 raise ValueError(
-                    ("The 'platforms' keyword must be defined for base %r. ", base)
+                    f"'platforms' keyword is not supported for base {base!r}. "
+                    "Use 'architectures' keyword instead."
                 )
-        else:
+            # set default value
             if not values.get("architectures"):
                 values["architectures"] = [get_host_architecture()]
-            if values.get("platforms"):
+        else:
+            if values.get("architectures"):
                 raise ValueError(
-                    "'platforms' keyword is not supported with 'base=core22'"
+                    f"'architectures' keyword is not supported for base {base!r}. "
+                    "Use 'platforms' keyword instead."
                 )
+            # set default value
+            if not values.get("platforms"):
+                values["platforms"] = {get_host_architecture(): None}
 
         return values
 
diff --git a/spread.yaml b/spread.yaml
index 121845e06a..166b7f2daa 100644
--- a/spread.yaml
+++ b/spread.yaml
@@ -272,10 +272,6 @@ suites:
    systems:
      - ubuntu-22.04*
 
- # TODO:
- # - add `platforms` keyword to all core24 snapcraft.yaml files
- # - add core24 platforms tests
- # - remove core24 architecture tests
  tests/spread/core24/:
    summary: core24 specific tests
    systems:
diff --git a/tests/spread/core24/grammar/snap/snapcraft.yaml b/tests/spread/core24/grammar/snap/snapcraft.yaml
index 7d489d89bc..cc23e71fc2 100644
--- a/tests/spread/core24/grammar/snap/snapcraft.yaml
+++ b/tests/spread/core24/grammar/snap/snapcraft.yaml
@@ -3,14 +3,16 @@ version: "1.0"
 summary: test
 description: |
   Exercise snapcraft's advanced grammar keywords, `on`, and `to`.
-  This test leverages architecture keywords, `build-on` and `build-for`.
+  This test leverages the platform keywords, `build-on` and `build-for`.
 grade: devel
 confinement: strict
 base: core22
-architectures:
-  - build-on: amd64
+platforms:
+  platform1:
+    build-on: amd64
     build-for: amd64
-  - build-on: amd64
+  platform2:
+    build-on: amd64
     build-for: arm64
 
 parts:
diff --git a/tests/spread/core24/package-repositories/test-foreign-armhf/snapcraft.yaml b/tests/spread/core24/package-repositories/test-foreign-armhf/snapcraft.yaml
index 21cd4dbe9c..e19ed458a4 100644
--- a/tests/spread/core24/package-repositories/test-foreign-armhf/snapcraft.yaml
+++ b/tests/spread/core24/package-repositories/test-foreign-armhf/snapcraft.yaml
@@ -6,8 +6,8 @@ grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: amd64
+platforms:
+  amd64:
 
 package-repositories:
   # The repo that contains libpython3.11-minimal:armhf
diff --git a/tests/spread/core24/package-repositories/test-foreign-i386/snapcraft.yaml b/tests/spread/core24/package-repositories/test-foreign-i386/snapcraft.yaml
index 4d1b23e4fc..43de7758e0 100644
--- a/tests/spread/core24/package-repositories/test-foreign-i386/snapcraft.yaml
+++ b/tests/spread/core24/package-repositories/test-foreign-i386/snapcraft.yaml
@@ -6,8 +6,8 @@ grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: amd64
+platforms:
+  amd64:
 
 package-repositories:
   # The repo that contains libpython3.11-minimal:i386
diff --git a/tests/spread/core24/architectures/snaps/arg-match/arguments.txt b/tests/spread/core24/platforms/snaps/arg-match/arguments.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/arg-match/arguments.txt
rename to tests/spread/core24/platforms/snaps/arg-match/arguments.txt
diff --git a/tests/spread/core24/architectures/snaps/arg-match/expected-snaps.txt b/tests/spread/core24/platforms/snaps/arg-match/expected-snaps.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/arg-match/expected-snaps.txt
rename to tests/spread/core24/platforms/snaps/arg-match/expected-snaps.txt
diff --git a/tests/spread/core24/architectures/snaps/arg-match/snap/snapcraft.yaml b/tests/spread/core24/platforms/snaps/arg-match/snap/snapcraft.yaml
similarity index 78%
rename from tests/spread/core24/architectures/snaps/arg-match/snap/snapcraft.yaml
rename to tests/spread/core24/platforms/snaps/arg-match/snap/snapcraft.yaml
index fe39cb03d7..8255bb1482 100644
--- a/tests/spread/core24/architectures/snaps/arg-match/snap/snapcraft.yaml
+++ b/tests/spread/core24/platforms/snaps/arg-match/snap/snapcraft.yaml
@@ -8,10 +8,12 @@ grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: amd64
+platforms:
+  platform1:
+    build-on: amd64
     build-for: amd64
-  - build-on: [amd64, arm64]
+  platform2:
+    build-on: [amd64, arm64]
     build-for: arm64
 
 parts:
diff --git a/tests/spread/core24/architectures/snaps/arg-no-match/arguments.txt b/tests/spread/core24/platforms/snaps/arg-no-match/arguments.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/arg-no-match/arguments.txt
rename to tests/spread/core24/platforms/snaps/arg-no-match/arguments.txt
diff --git a/tests/spread/core24/architectures/snaps/arg-no-match/expected-failure.txt b/tests/spread/core24/platforms/snaps/arg-no-match/expected-failure.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/arg-no-match/expected-failure.txt
rename to tests/spread/core24/platforms/snaps/arg-no-match/expected-failure.txt
diff --git a/tests/spread/core24/architectures/snaps/arg-no-match/expected-snaps.txt b/tests/spread/core24/platforms/snaps/arg-no-match/expected-snaps.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/arg-no-match/expected-snaps.txt
rename to tests/spread/core24/platforms/snaps/arg-no-match/expected-snaps.txt
diff --git a/tests/spread/core24/architectures/snaps/arg-no-match/snap/snapcraft.yaml b/tests/spread/core24/platforms/snaps/arg-no-match/snap/snapcraft.yaml
similarity index 78%
rename from tests/spread/core24/architectures/snaps/arg-no-match/snap/snapcraft.yaml
rename to tests/spread/core24/platforms/snaps/arg-no-match/snap/snapcraft.yaml
index c83aef8d53..f66ec11694 100644
--- a/tests/spread/core24/architectures/snaps/arg-no-match/snap/snapcraft.yaml
+++ b/tests/spread/core24/platforms/snaps/arg-no-match/snap/snapcraft.yaml
@@ -8,10 +8,12 @@ grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: amd64
+platforms:
+  platform1:
+    build-on: amd64
     build-for: amd64
-  - build-on: [amd64, arm64]
+  platform2:
+    build-on: [amd64, arm64]
     build-for: arm64
 
 parts:
diff --git a/tests/spread/core24/architectures/snaps/build-on-no-match/expected-failure.txt b/tests/spread/core24/platforms/snaps/build-on-no-match/expected-failure.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/build-on-no-match/expected-failure.txt
rename to tests/spread/core24/platforms/snaps/build-on-no-match/expected-failure.txt
diff --git a/tests/spread/core24/architectures/snaps/build-on-no-match/expected-snaps.txt b/tests/spread/core24/platforms/snaps/build-on-no-match/expected-snaps.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/build-on-no-match/expected-snaps.txt
rename to tests/spread/core24/platforms/snaps/build-on-no-match/expected-snaps.txt
diff --git a/tests/spread/core24/architectures/snaps/build-on-no-match/snap/snapcraft.yaml b/tests/spread/core24/platforms/snaps/build-on-no-match/snap/snapcraft.yaml
similarity index 78%
rename from tests/spread/core24/architectures/snaps/build-on-no-match/snap/snapcraft.yaml
rename to tests/spread/core24/platforms/snaps/build-on-no-match/snap/snapcraft.yaml
index ab0bcb19d3..6fc3a674cc 100644
--- a/tests/spread/core24/architectures/snaps/build-on-no-match/snap/snapcraft.yaml
+++ b/tests/spread/core24/platforms/snaps/build-on-no-match/snap/snapcraft.yaml
@@ -8,10 +8,12 @@ grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: arm64
+platforms:
+  platform1:
+    build-on: arm64
     build-for: arm64
-  - build-on: armhf
+  platform2:
+    build-on: armhf
     build-for: armhf
 
 parts:
diff --git a/tests/spread/core24/architectures/snaps/env-var-match/environmental-variables.txt b/tests/spread/core24/platforms/snaps/env-var-match/environmental-variables.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/env-var-match/environmental-variables.txt
rename to tests/spread/core24/platforms/snaps/env-var-match/environmental-variables.txt
diff --git a/tests/spread/core24/architectures/snaps/env-var-match/expected-snaps.txt b/tests/spread/core24/platforms/snaps/env-var-match/expected-snaps.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/env-var-match/expected-snaps.txt
rename to tests/spread/core24/platforms/snaps/env-var-match/expected-snaps.txt
diff --git a/tests/spread/core24/architectures/snaps/env-var-match/snap/snapcraft.yaml b/tests/spread/core24/platforms/snaps/env-var-match/snap/snapcraft.yaml
similarity index 78%
rename from tests/spread/core24/architectures/snaps/env-var-match/snap/snapcraft.yaml
rename to tests/spread/core24/platforms/snaps/env-var-match/snap/snapcraft.yaml
index a4f703a275..ff493e4d47 100644
--- a/tests/spread/core24/architectures/snaps/env-var-match/snap/snapcraft.yaml
+++ b/tests/spread/core24/platforms/snaps/env-var-match/snap/snapcraft.yaml
@@ -8,10 +8,12 @@ grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: amd64
+platforms:
+  platform1:
+    build-on: amd64
     build-for: amd64
-  - build-on: [amd64, arm64]
+  platform2:
+    build-on: [amd64, arm64]
     build-for: arm64
 
 parts:
diff --git a/tests/spread/core24/architectures/snaps/env-var-no-match/environmental-variables.txt b/tests/spread/core24/platforms/snaps/env-var-no-match/environmental-variables.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/env-var-no-match/environmental-variables.txt
rename to tests/spread/core24/platforms/snaps/env-var-no-match/environmental-variables.txt
diff --git a/tests/spread/core24/architectures/snaps/env-var-no-match/expected-failure.txt b/tests/spread/core24/platforms/snaps/env-var-no-match/expected-failure.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/env-var-no-match/expected-failure.txt
rename to tests/spread/core24/platforms/snaps/env-var-no-match/expected-failure.txt
diff --git a/tests/spread/core24/architectures/snaps/env-var-no-match/expected-snaps.txt b/tests/spread/core24/platforms/snaps/env-var-no-match/expected-snaps.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/env-var-no-match/expected-snaps.txt
rename to tests/spread/core24/platforms/snaps/env-var-no-match/expected-snaps.txt
diff --git a/tests/spread/core24/architectures/snaps/env-var-no-match/snap/snapcraft.yaml b/tests/spread/core24/platforms/snaps/env-var-no-match/snap/snapcraft.yaml
similarity index 78%
rename from tests/spread/core24/architectures/snaps/env-var-no-match/snap/snapcraft.yaml
rename to tests/spread/core24/platforms/snaps/env-var-no-match/snap/snapcraft.yaml
index d94a202809..e95d04c840 100644
--- a/tests/spread/core24/architectures/snaps/env-var-no-match/snap/snapcraft.yaml
+++ b/tests/spread/core24/platforms/snaps/env-var-no-match/snap/snapcraft.yaml
@@ -8,10 +8,12 @@ grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: amd64
+platforms:
+  platform1:
+    build-on: amd64
     build-for: amd64
-  - build-on: [amd64, arm64]
+  platform2:
+    build-on: [amd64, arm64]
     build-for: arm64
 
 parts:
diff --git a/tests/spread/core24/architectures/snaps/multiple-build-for/expected-failure.txt b/tests/spread/core24/platforms/snaps/multiple-build-for/expected-failure.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/multiple-build-for/expected-failure.txt
rename to tests/spread/core24/platforms/snaps/multiple-build-for/expected-failure.txt
diff --git a/tests/spread/core24/architectures/snaps/multiple-build-for/expected-snaps.txt b/tests/spread/core24/platforms/snaps/multiple-build-for/expected-snaps.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/multiple-build-for/expected-snaps.txt
rename to tests/spread/core24/platforms/snaps/multiple-build-for/expected-snaps.txt
diff --git a/tests/spread/core24/architectures/snaps/multiple-build-for/snap/snapcraft.yaml b/tests/spread/core24/platforms/snaps/multiple-build-for/snap/snapcraft.yaml
similarity index 64%
rename from tests/spread/core24/architectures/snaps/multiple-build-for/snap/snapcraft.yaml
rename to tests/spread/core24/platforms/snaps/multiple-build-for/snap/snapcraft.yaml
index 948701770b..ba1a31906c 100644
--- a/tests/spread/core24/architectures/snaps/multiple-build-for/snap/snapcraft.yaml
+++ b/tests/spread/core24/platforms/snaps/multiple-build-for/snap/snapcraft.yaml
@@ -7,11 +7,13 @@ grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: amd64
-    build-for: amd64
-  - build-on: [amd64, arm64]
-    build-for: arm64
+platforms:
+  platform1:
+    build-on: [amd64]
+    build-for: [amd64]
+  platform2:
+    build-on: [amd64, arm64]
+    build-for: [arm64]
 
 parts:
   nil:
diff --git a/tests/spread/core24/architectures/snaps/single-arch/expected-snaps.txt b/tests/spread/core24/platforms/snaps/single-arch/expected-snaps.txt
similarity index 100%
rename from tests/spread/core24/architectures/snaps/single-arch/expected-snaps.txt
rename to tests/spread/core24/platforms/snaps/single-arch/expected-snaps.txt
diff --git a/tests/spread/core24/architectures/snaps/single-arch/snap/snapcraft.yaml b/tests/spread/core24/platforms/snaps/single-arch/snap/snapcraft.yaml
similarity index 54%
rename from tests/spread/core24/architectures/snaps/single-arch/snap/snapcraft.yaml
rename to tests/spread/core24/platforms/snaps/single-arch/snap/snapcraft.yaml
index f8e1c31e60..7e082a42ab 100644
--- a/tests/spread/core24/architectures/snaps/single-arch/snap/snapcraft.yaml
+++ b/tests/spread/core24/platforms/snaps/single-arch/snap/snapcraft.yaml
@@ -2,14 +2,15 @@ name: single-arch
 version: "1.0"
 summary: test
 description: |
-  A single architecture should create a single snap file.
+  A single platform should create a single snap file.
 grade: devel
 confinement: strict
 base: core24
 build-base: devel
-architectures:
-  - build-on: amd64
-    build-for: amd64
+platforms:
+  platform1:
+    build-on: [amd64, arm64]
+    build-for: [amd64]
 
 parts:
   nil:
diff --git a/tests/spread/core24/architectures/task.yaml b/tests/spread/core24/platforms/task.yaml
similarity index 100%
rename from tests/spread/core24/architectures/task.yaml
rename to tests/spread/core24/platforms/task.yaml
diff --git a/tests/unit/conftest.py b/tests/unit/conftest.py
index 7685bc7755..0e975bd56c 100644
--- a/tests/unit/conftest.py
+++ b/tests/unit/conftest.py
@@ -397,41 +397,18 @@ def default_project(extra_project_params):
         grade="devel",
         parts=parts,
         license="MIT",
-        platforms={"amd64": {"build-on": ["amd64"], "build-for": ["amd64"]}},
         **extra_project_params,
     )
 
 
 @pytest.fixture()
-def default_project_core22(extra_project_params):
-    from craft_application.models import SummaryStr, VersionStr
-
-    from snapcraft.models.project import Project
-
-    parts = extra_project_params.pop("parts", {})
-
-    return Project(
-        name="default",
-        version=VersionStr("1.0"),
-        summary=SummaryStr("default project"),
-        description="default project",
-        base="core22",
-        build_base="devel",
-        grade="devel",
-        parts=parts,
-        license="MIT",
-        **extra_project_params,
-    )
-
-
-@pytest.fixture()
-def default_factory(default_project_core22):
+def default_factory(default_project):
     from snapcraft.application import APP_METADATA
     from snapcraft.services import SnapcraftServiceFactory
 
     factory = SnapcraftServiceFactory(
         app=APP_METADATA,
-        project=default_project_core22,
+        project=default_project,
     )
     return factory
 
diff --git a/tests/unit/models/test_projects.py b/tests/unit/models/test_projects.py
index a585c7a210..09acefdcec 100644
--- a/tests/unit/models/test_projects.py
+++ b/tests/unit/models/test_projects.py
@@ -1,6 +1,6 @@
 # -*- Mode:Python; indent-tabs-mode:nil; tab-width:4 -*-
 #
-# Copyright 2022-2023 Canonical Ltd.
+# Copyright 2022-2024 Canonical Ltd.
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 3 as
@@ -14,24 +14,30 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import itertools
 from typing import Any, Dict, cast
 
 import pydantic
 import pytest
+from craft_application.errors import CraftValidationError
 from craft_application.models import UniqueStrList
 
-from snapcraft import errors
+from snapcraft import const, errors
 from snapcraft.models import (
     MANDATORY_ADOPTABLE_FIELDS,
     Architecture,
     ContentPlug,
     GrammarAwareProject,
     Hook,
+    Platform,
     Project,
 )
 from snapcraft.models.project import apply_root_packages
 from snapcraft.utils import get_host_architecture
 
+# required project data for core24 snaps
+CORE24_DATA = {"base": "core24", "build_base": "devel", "grade": "devel"}
+
 
 @pytest.fixture
 def project_yaml_data():
@@ -579,14 +585,7 @@ def test_project_development_base_error(self, project_yaml_data):
         error = "build-base must be 'devel' when base is 'core24'"
 
         with pytest.raises(errors.ProjectValidationError, match=error):
-            Project.unmarshal(
-                project_yaml_data(
-                    base="core24",
-                    platforms={
-                        "amd64v2": {"build-on": ["amd64"], "build-for": ["amd64"]}
-                    },
-                )
-            )
+            Project.unmarshal(project_yaml_data(base="core24"))
 
     def test_project_global_plugs_warning(self, project_yaml_data, emitter):
         data = project_yaml_data(plugs={"desktop": None, "desktop-legacy": None})
@@ -669,6 +668,57 @@ def test_project_hooks_plugs_empty(self, project_yaml_data):
             Project.unmarshal(project_yaml_data(hooks=hook))
 
 
+class TestPlatforms:
+    """Validate platforms."""
+
+    VALID_PLATFORM_ARCHITECTURES = [
+        # single architecture in a list
+        *(list(x) for x in itertools.combinations(const.SnapArch, 1)),
+        # two architectures in a list
+        *(list(x) for x in itertools.combinations(const.SnapArch, 2)),
+    ]
+
+    @pytest.mark.parametrize("build_on", VALID_PLATFORM_ARCHITECTURES)
+    @pytest.mark.parametrize("build_for", [[arch] for arch in const.SnapArch])
+    def test_platform_validation_lists(self, build_on, build_for, project_yaml_data):
+        """Unmarshal build-on and build-for lists."""
+        platform_data = Platform(**{"build-on": build_on, "build-for": build_for})
+
+        assert platform_data.build_for == build_for
+        assert platform_data.build_on == build_on
+
+    @pytest.mark.parametrize("build_on", const.SnapArch)
+    @pytest.mark.parametrize("build_for", const.SnapArch)
+    def test_platform_validation_strings(self, build_on, build_for, project_yaml_data):
+        """Unmarshal and vectorize build-on and build-for strings."""
+        platform_data = Platform(**{"build-on": build_on, "build-for": build_for})
+
+        assert platform_data.build_for == [build_for]
+        assert platform_data.build_on == [build_on]
+
+    def test_platform_build_for_requires_build_on(self, project_yaml_data):
+        """Raise an error if build-for is provided by build-on is not."""
+        with pytest.raises(CraftValidationError) as raised:
+            Platform(**{"build-for": [const.SnapArch.amd64]})
+
+        assert "'build_for' expects 'build_on' to also be provided" in str(raised.value)
+
+    def test_platform_default(self, project_yaml_data):
+        """Default value for platforms is the host architecture."""
+        project = Project.unmarshal(project_yaml_data(**CORE24_DATA))
+
+        assert project.platforms == {get_host_architecture(): None}
+
+    def test_platforms_not_allowed_core22(self, project_yaml_data):
+        with pytest.raises(errors.ProjectValidationError) as raised:
+            Project.unmarshal(project_yaml_data(platforms={"amd64": None}))
+
+        assert (
+            "'platforms' keyword is not supported for base 'core22'. "
+            "Use 'architectures' keyword instead." in str(raised.value)
+        )
+
+
 class TestAppValidation:
     """Validate apps."""
 
@@ -1759,6 +1809,16 @@ def test_project_get_build_for_arch_triplet_all(self, project_yaml_data):
 
         assert not arch_triplet
 
+    def test_architectures_not_allowed(self, project_yaml_data):
+        """'architectures' keyword is not allowed if base is not core22."""
+        with pytest.raises(errors.ProjectValidationError) as raised:
+            Project.unmarshal(project_yaml_data(**CORE24_DATA, architectures=["amd64"]))
+
+        assert (
+            "'architectures' keyword is not supported for base 'core24'. "
+            "Use 'platforms' keyword instead."
+        ) in str(raised.value)
+
 
 class TestApplyRootPackages:
     """Test Transform the Project."""
diff --git a/tests/unit/test_application.py b/tests/unit/test_application.py
index 99d4f72bdf..d3e813cf5b 100644
--- a/tests/unit/test_application.py
+++ b/tests/unit/test_application.py
@@ -19,10 +19,11 @@
 from textwrap import dedent
 
 import pytest
-from craft_providers import bases
+from craft_application.errors import CraftValidationError
+from craft_application.models import BuildInfo
+from craft_providers.bases import BaseName
 
 from snapcraft import application, services
-from snapcraft.const import SnapArch
 from snapcraft.models.project import Architecture
 
 
@@ -38,25 +39,160 @@ def architectures(request):
     return request.param
 
 
-def test_build_planner_success_base_only():
-    data = {
-        "base": "core24",
-        "name": "test",
-        "platforms": {"amd64v2": {"build-on": ["amd64"], "build-for": ["amd64"]}},
-    }
+@pytest.mark.parametrize(
+    ("platforms", "expected_build_infos"),
+    [
+        pytest.param(
+            {"amd64": None},
+            [
+                BuildInfo(
+                    build_on="amd64",
+                    build_for="amd64",
+                    base=BaseName(name="ubuntu", version="24.04"),
+                    platform="amd64",
+                )
+            ],
+            id="single_platform_as_arch",
+        ),
+        pytest.param(
+            {
+                "arm64": {
+                    "build-on": ["arm64", "armhf"],
+                    "build-for": ["arm64"],
+                },
+            },
+            [
+                BuildInfo(
+                    build_on="arm64",
+                    build_for="arm64",
+                    base=BaseName(name="ubuntu", version="24.04"),
+                    platform="arm64",
+                ),
+                BuildInfo(
+                    build_on="armhf",
+                    build_for="arm64",
+                    base=BaseName(name="ubuntu", version="24.04"),
+                    platform="arm64",
+                ),
+            ],
+            id="multiple_build_on",
+        ),
+        pytest.param(
+            {
+                "amd64v2": {
+                    "build-on": ["amd64"],
+                    "build-for": "amd64",
+                },
+            },
+            [
+                BuildInfo(
+                    build_on="amd64",
+                    build_for="amd64",
+                    base=BaseName(name="ubuntu", version="24.04"),
+                    platform="amd64v2",
+                )
+            ],
+            id="custom_platform_name",
+        ),
+    ],
+)
+def test_build_planner_get_build_plan(platforms, expected_build_infos):
+    """Test `get_build_plan()` function with different platforms."""
+    planner = application.SnapcraftBuildPlanner.parse_obj(
+        {"name": "test-snap", "base": "core24", "platforms": platforms}
+    )
+
+    actual_build_infos = planner.get_build_plan()
+
+    assert actual_build_infos == expected_build_infos
+
+
+def test_build_planner_get_build_plan_base(mocker):
+    """Test `get_build_plan()` uses the correct base."""
+    mock_get_effective_base = mocker.patch(
+        "snapcraft.application.get_effective_base", return_value="core24"
+    )
+    planner = application.SnapcraftBuildPlanner.parse_obj(
+        {
+            "name": "test-snap",
+            "base": "test-base",
+            "build-base": "test-build-base",
+            "platforms": {"amd64": None},
+            "project_type": "test-type",
+        }
+    )
+
+    actual_build_infos = planner.get_build_plan()
+
+    assert actual_build_infos == [
+        BuildInfo(
+            platform="amd64",
+            build_on="amd64",
+            build_for="amd64",
+            base=BaseName(name="ubuntu", version="24.04"),
+        )
+    ]
+    mock_get_effective_base.assert_called_once_with(
+        base="test-base",
+        build_base="test-build-base",
+        project_type="test-type",
+        name="test-snap",
+    )
+
+
+def test_project_platform_error_has_context():
+    """Platform validation errors include which platform entry is invalid."""
+    with pytest.raises(CraftValidationError) as raised:
+        application.SnapcraftBuildPlanner.parse_obj(
+            {
+                "name": "test-snap",
+                "base": "test-base",
+                "build-base": "test-build-base",
+                "platforms": {"test-platform": {"build-for": ["amd64"]}},
+                "project_type": "test-type",
+            }
+        )
 
-    planner = application.SnapcraftBuildPlanner.parse_obj(data)
+    assert "'test-platform': 'build_for' expects 'build_on'" in str(raised.value)
 
-    actual = planner.get_build_plan()
 
-    for build_info in actual:
-        assert build_info.base == bases.BaseName("ubuntu", "24.04")
-        assert build_info.build_for == SnapArch.amd64
-        assert build_info.build_on == SnapArch.amd64
-        assert build_info.platform == "amd64v2"
+def test_project_platform_mismatch():
+    """Raise an error if platform name and build-for are valid but different archs."""
+    with pytest.raises(CraftValidationError) as raised:
+        application.SnapcraftBuildPlanner.parse_obj(
+            {
+                "name": "test-snap",
+                "base": "test-base",
+                "build-base": "test-build-base",
+                "platforms": {"amd64": {"build-on": ["amd64"], "build-for": ["arm64"]}},
+                "project_type": "test-type",
+            }
+        )
+
+    assert (
+        "if 'build_for' is provided and the platform entry label "
+        "corresponds to a valid architecture, then both values must match. "
+        "amd64 != arm64" in str(raised.value)
+    )
 
 
-# TODO: add tests `get_build_plan()`
+def test_project_platform_unknown_name():
+    """Raise an error if an empty platform is not a valid architecture."""
+    with pytest.raises(CraftValidationError) as raised:
+        application.SnapcraftBuildPlanner.parse_obj(
+            {
+                "name": "test-snap",
+                "base": "test-base",
+                "build-base": "test-build-base",
+                "platforms": {"unknown": None},
+                "project_type": "test-type",
+            }
+        )
+
+    assert (
+        "platform entry label must correspond to a valid architecture "
+        "if 'build-for' is not provided." in str(raised.value)
+    )
 
 
 @pytest.mark.parametrize("env_vars", application.MAPPED_ENV_VARS.items())
@@ -110,11 +246,7 @@ def test_application_expand_extensions(emitter, monkeypatch, extension_source, n
             base: core24
             build-base: devel
             platforms:
-                amd64:
-                    build-on:
-                    - amd64
-                    build-for:
-                    - amd64
+                amd64: null
             license: MIT
             parts:
                 fake-extension/fake-part: