This is a mod for the game cavestory. Unlike most games a mod to this game is not a simple. Cavestory is small enough and simple enough there is an entire language dedicated to editing it. This editor is so small it is included in the files.
It's called cave editor amongst other things. This so far only uses this language and not straight assembly. But, I'm hoping to code in that soon.
17 Yamashita Farm (Plant) [Shadow Entrance]
30 Grasstown Hut (WeedB) [Shadow Doors]
44 Boulder Chamber (MazeS) [Shadow Doors]
More not listed...