Today I got some code down, the main things work in the dummy test stage. Enought to put all parts on the screen for a good impression where this may go.
Here is a first screenshot:
Moving and playing notes works, matching songs with the melody and arrows to indicate the sequence works, random shrubbery works.
Monsters are in the making, I coded the scheduling timer described at RogueBasin already in to take care of different movement speed of monsters.