like a game where you can open its level editor, make a level, and release it as a standalone file that runs with that game. some examples that i already know:
- DOOM
- Quake
- Duke Nukem 3D
- Half-Life
- Half-Life 2
i'm looking for more examples because i wanna try and improve my FPS level design skills by using an existing game's design toolkit, so i don't have to build a whole new game just to design some cool levels.






