That's genius!
Tower defense and bullet hell work so well together I'm surprised I haven't seen a game like this before. Everything in the game is excellent.
One really nitpicky detail is that I feel like the music is really good, but it doesn't fit the game at all. It's all technological and brass hits and random vocal clips, “system malfunction”... The feeling it invokes is of something computer-y and futuristic, which isn't the feel I got from playing, it feels like the music wasn't made for this game.
This is really minor though and doesn't affect the score I gave you at all, just something I wanted to note.