diff --git a/AGENTS.md b/AGENTS.md
index 3a9efd8..6ff7b51 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -15,7 +15,6 @@
   - `src/application`
   - `src/domain`
   - `src/engine`
-- Each layer may further split into `include/` and `src/` under its own root.
 - When touching build files, confirm that referenced source files are actually tracked in Git.
 
 ## Build
@@ -29,7 +28,8 @@
 
 ## Source Layout
 - All C++ source files live under `src/`.
-- Split each layer so headers live under `src/<layer>/include/<layer>/` and source files live under `src/<layer>/src/`.
+- Keep each layer's headers and source files directly under `src/<layer>/` unless a clear need for subfolders appears.
+- Use `src/` as the include root.
 - Preferred includes:
   - `#include "application/..."`
   - `#include "domain/..."`
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9d56db7..3247c35 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -32,33 +32,35 @@ function(configure_project_target target_name)
     set_property(TARGET ${target_name} PROPERTY VS_DEBUGGER_WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}")
 endfunction()
 
+set(SAFECROWD_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}/src")
+
 add_library(ecs_engine STATIC
-    src/engine/include/engine/EngineConfig.h
-    src/engine/include/engine/EngineRuntime.h
-    src/engine/include/engine/EngineState.h
-    src/engine/include/engine/EngineStats.h
-    src/engine/include/engine/EngineStepContext.h
-    src/engine/include/engine/EngineSystem.h
-    src/engine/include/engine/FrameClock.h
-    src/engine/src/EngineRuntime.cpp
-    src/engine/src/FrameClock.cpp
+    src/engine/EngineConfig.h
+    src/engine/EngineRuntime.h
+    src/engine/EngineState.h
+    src/engine/EngineStats.h
+    src/engine/EngineStepContext.h
+    src/engine/EngineSystem.h
+    src/engine/FrameClock.h
+    src/engine/EngineRuntime.cpp
+    src/engine/FrameClock.cpp
 )
 
 target_include_directories(ecs_engine
     PUBLIC
-        ${CMAKE_CURRENT_SOURCE_DIR}/src/engine/include
+        ${SAFECROWD_SRC_DIR}
 )
 
 configure_project_target(ecs_engine)
 
 add_library(safecrowd_domain STATIC
-    src/domain/include/domain/SafeCrowdDomain.h
-    src/domain/src/SafeCrowdDomain.cpp
+    src/domain/SafeCrowdDomain.h
+    src/domain/SafeCrowdDomain.cpp
 )
 
 target_include_directories(safecrowd_domain
     PUBLIC
-        ${CMAKE_CURRENT_SOURCE_DIR}/src/domain/include
+        ${SAFECROWD_SRC_DIR}
 )
 
 target_link_libraries(safecrowd_domain
@@ -94,14 +96,14 @@ endif()
 
 if (SAFECROWD_BUILD_APP)
     add_executable(safecrowd_app
-        src/application/include/application/MainWindow.h
-        src/application/src/main.cpp
-        src/application/src/MainWindow.cpp
+        src/application/MainWindow.h
+        src/application/main.cpp
+        src/application/MainWindow.cpp
     )
 
     target_include_directories(safecrowd_app
         PRIVATE
-            ${CMAKE_CURRENT_SOURCE_DIR}/src/application/include
+            ${SAFECROWD_SRC_DIR}
     )
 
     set_target_properties(safecrowd_app PROPERTIES
diff --git "a/docs/\355\224\204\353\241\234\354\240\235\355\212\270 \352\265\254\354\241\260.md" "b/docs/\355\224\204\353\241\234\354\240\235\355\212\270 \352\265\254\354\241\260.md"
index 5071cd2..0f07534 100644
--- "a/docs/\355\224\204\353\241\234\354\240\235\355\212\270 \352\265\254\354\241\260.md"	
+++ "b/docs/\355\224\204\353\241\234\354\240\235\355\212\270 \352\265\254\354\241\260.md"	
@@ -44,14 +44,8 @@ Project/
   vcpkg.json
   src/
     engine/
-      include/engine/
-      src/
     domain/
-      include/domain/
-      src/
     application/
-      include/application/
-      src/
   external/
   docs/
 ```
@@ -180,12 +174,12 @@ Qt viewport가 엔진 렌더러와 직접 연결되어야 하면 `application ->
 
 ### 7.1. 소스 루트 원칙
 - 모든 C++ 소스와 헤더는 `src/` 아래에 둔다.
-- 각 계층은 `src/<layer>/include/<layer>/`에 헤더를, `src/<layer>/src/`에 구현 파일을 둔다.
+- 각 계층의 헤더와 구현 파일은 기본적으로 `src/<layer>/` 바로 아래에 둔다.
 - 따라서 include는 `#include "engine/..."`, `#include "domain/..."`, `#include "application/..."` 형태를 사용한다.
 
 ### 7.2. CMake 원칙
 - `CMakeLists.txt`의 타깃 소스 경로는 항상 `src/...`를 기준으로 적는다.
-- 각 타깃의 include directory는 해당 계층의 `src/<layer>/include/`를 공개 루트로 잡는다.
+- include directory는 `src/`를 공통 공개 루트로 잡는다.
 - 계층이 늘어나도 당분간 루트 `CMakeLists.txt` 하나에서 관리하고, 타깃 수와 파일 수가 커질 때만 하위 `CMakeLists.txt`를 추가한다.
 
 ### 7.3. 외부 의존성 원칙
diff --git a/src/application/src/MainWindow.cpp b/src/application/MainWindow.cpp
similarity index 100%
rename from src/application/src/MainWindow.cpp
rename to src/application/MainWindow.cpp
diff --git a/src/application/include/application/MainWindow.h b/src/application/MainWindow.h
similarity index 100%
rename from src/application/include/application/MainWindow.h
rename to src/application/MainWindow.h
diff --git a/src/application/src/main.cpp b/src/application/main.cpp
similarity index 100%
rename from src/application/src/main.cpp
rename to src/application/main.cpp
diff --git a/src/domain/src/SafeCrowdDomain.cpp b/src/domain/SafeCrowdDomain.cpp
similarity index 100%
rename from src/domain/src/SafeCrowdDomain.cpp
rename to src/domain/SafeCrowdDomain.cpp
diff --git a/src/domain/include/domain/SafeCrowdDomain.h b/src/domain/SafeCrowdDomain.h
similarity index 100%
rename from src/domain/include/domain/SafeCrowdDomain.h
rename to src/domain/SafeCrowdDomain.h
diff --git a/src/engine/include/engine/EngineConfig.h b/src/engine/EngineConfig.h
similarity index 100%
rename from src/engine/include/engine/EngineConfig.h
rename to src/engine/EngineConfig.h
diff --git a/src/engine/src/EngineRuntime.cpp b/src/engine/EngineRuntime.cpp
similarity index 100%
rename from src/engine/src/EngineRuntime.cpp
rename to src/engine/EngineRuntime.cpp
diff --git a/src/engine/include/engine/EngineRuntime.h b/src/engine/EngineRuntime.h
similarity index 100%
rename from src/engine/include/engine/EngineRuntime.h
rename to src/engine/EngineRuntime.h
diff --git a/src/engine/include/engine/EngineState.h b/src/engine/EngineState.h
similarity index 100%
rename from src/engine/include/engine/EngineState.h
rename to src/engine/EngineState.h
diff --git a/src/engine/include/engine/EngineStats.h b/src/engine/EngineStats.h
similarity index 100%
rename from src/engine/include/engine/EngineStats.h
rename to src/engine/EngineStats.h
diff --git a/src/engine/include/engine/EngineStepContext.h b/src/engine/EngineStepContext.h
similarity index 100%
rename from src/engine/include/engine/EngineStepContext.h
rename to src/engine/EngineStepContext.h
diff --git a/src/engine/include/engine/EngineSystem.h b/src/engine/EngineSystem.h
similarity index 100%
rename from src/engine/include/engine/EngineSystem.h
rename to src/engine/EngineSystem.h
diff --git a/src/engine/src/FrameClock.cpp b/src/engine/FrameClock.cpp
similarity index 100%
rename from src/engine/src/FrameClock.cpp
rename to src/engine/FrameClock.cpp
diff --git a/src/engine/include/engine/FrameClock.h b/src/engine/FrameClock.h
similarity index 100%
rename from src/engine/include/engine/FrameClock.h
rename to src/engine/FrameClock.h