Thanks, glad you like it! The game isn't open source, but you don't actually need to change any of the code to add more maps, you can just use the built in modding support for that. If you're interested in how my game was made, maps are midis, I used this godot addon https://github.com/nlaha/godot-midi , which plays the midis in sync with the music, spawning notes at the right time.