Nice concept which fits the theme of limited space, with the circle reducing in size as you get hit and i also like how it increases if you take out enemies! I like the shooting sfx and the frequency of shooting, felt nice to shoot fast!
I have a few suggestions which could possibly improve the experience: 1. I think if you could start with fewer circles initially and then gradually increase the number, it would help the player get adjusted to the enemy movements. I think the difficulty is a bit high to start with! 2. Adding some visual feedback for circles disappearing or the player getting hit would add a lot more juice.. just a few camera shakes here and there, some particle effects.. woud go a long way!
Overall good game, nice use of the shapes and the theme!