Really good design and impressive graphics!
I will try to not repeat what others have said already. There are two things that I'd add to the game: a FPS limiter setting which also could help with preventing the low frame drops (talking about the 0.1% fps spikes when in example effects pop, or enemies are instanced), and the other thing that i'd suggest is giving the camera a bit more distance from the ground, because when the enemies come from the corners or the sides you have more than enough time to react, but when they come from straight down due to the camera angle, you don't.
It's a very well done game and impressive for such a short time period, cheers! ;)