Haha the twist was definitely unexpected :D
Although I'm not good enough to actually complete the last level, I think the extra gameplay mechanic makes this game very different from the generic piano tiles game that you initially disguised it as!
The evil menu looks very cool, is that the Easter egg? I don't actually know how I got it to appear lol, I just clicked the play again button and it came up.
The main thing that would make this a lot better is more rhythm rather than just having the notes come every beat. I kept playing it with the right rhythm on purpose to make it sound nicer and it would be really cool to have it play with the right rhythm. Although, I must admit you picked a great piece that still works pretty well even if you play every note with the same rhythm!
Lastly, the sounds are great (which is obviously very important for a rhythm game) and it is very obvious when you mess up which is good too.