Each Xth level folder needs these:

You need to put those to make it playable!

What are those?

levelX_map is representation of tiles in tileset with some IDs that forms a map. That IDs stored in include/BD_map.h. If you want your to have your own tileset and want to give their custom ID, modify the IDs' values and recompile. There are already good levels created by me, so you don't have to if you want to experience that.
levelX_musicY.wav is music that plays on that level. There is a timer and at some percentages of time next music starts to play. I used it to increase tension.
levelX_tileset.png is consisting from tiles with their different frames. World 5 has a different color key. level8 and level9 is in World 5. If you want to change color keys of levels go to the get_color_key function in src/level.c, modify and recompile it. I put a 2 templates representing how should you put your textures but in a blurred way because of legal problems. I also put an blurred_info_tileset.png to make it explicit what this tile is. Tiles below the info text are animation frames of that tile which is represented by the info text.