Fantastic art direction and sprite work. I was killed on contact with a trooper coming in through the left door. They appeared once I cleared the room as a next wave and spawned on top of me. What about placing a flashing "!" or something next to doors that will spawn the troopers for a brief moment before hand. It can act as a telegraph to avoid those insta- kill situations and the player can get out of the way in time.
Really nice arcade action with the screen shake, enemy AI and attention to detail on the sprite effects. Well done.