Great game and also really easy to understand. This concept could have gotten convoluted and or confusing but the way it was taught and laid out made it very easy to pickup. Nicely done. I also just think that this is a genre match made in heaven and I am surprised that I have not seen this mixture of wave based tower defense with rhythm game.
Feedback: I think perhaps more visual indication when an enemy is invulnerable to your attacks would be a good idea. Just because there are so many enemies on screen it can be easy to lose sight of how effective your attacks are or are not.
Fantastic idea and execution for a game, and also really great soundtrack. I had a lot of fun.