My main critique: the first room after the tutorial was really hard.
Part of that was due to me not being able to see the enemies, while they were still able to shoot at me. I'd recommend you zoom the camera out a lot, so that enemies can only target the player if they're currently on screen.
Concept was a good one, and I'm sure I'm going to see a lot of games in this genre.
The best part, I'd say, was your sound design along with game feel. There was just a little bit of screen shake, and that was enough to make shooting feel good.