There's a lot of different mechanics going on in this, quite impressive that you got so much done. Athough I knew I could be damaged outside the circle I forgot about it and didn't realise until too late. Perhaps add some secondary feedback when the health is getting low to get the players attention. I liked that there were two different kinds of enemies, some more damage feedback would make combat more satisfying. Finally I think increasing the spawn rate would make the game more exciting as you get more familiar with it. Otherwise it's a solid game loop with some great mechanics - I really enjoyed :D