I like the art, animation, simple mechanics and sound effects.
I found that if you stay in a corner or near one and just shoot, it's easier to not lose health. The enemies seem to spawn from multiple directions at random, it's actually easier to get hit when moving around. Maybe you could add a mechanic or rule to keep the player moving. (Just a suggestion)
Overall, a fun game and a cool idea. Congratulations on making your first game!