It's great how you manage to create visual variety with minimal resources using simple techniques (color changes, shaking).
The game is enjoyable, but the enemy behavior could use some work. Right now, they just all stay in a group and chase me. More enemy variety would greatly improve the game. Also, the shooting enemies appear too late.
Also, either I didn't notice, or it's not shown anywhere whether my weapon is loaded or not.
Anyway, it was fun. Good job!