The amount of juice you managed to pack into this game in just 3 days is amazing! The SFX are nice, although I wish there was music. Stars don't seem very useful. Replacing them with health/ammo pickups could add more motivation to grab them. Though the current amount of health the player has per level feels pretty good.
The enemies seem like they're hard-of-hearing the way you can shoot right next to them without them being alerted, but once they know your presence, they're fun. The map design feels rather redundant. Maybe it'd be more exciting if you couldn't see the enemies until they're in line-of-sight, or when you enter the room.
Sometimes the enemies will shoot backwards. After finishing the game and returning to the level select menu, the cursor is hidden. I like how you did the gun cooldown system.
Nice juicy game. Well done!