Great job.
I love almost everything about this game.
Level design is sound.
Mobs too. I like a lot that you can jump on zombies without getting hurt; and the purple line with eye.
Good tutorial (except missing wasd for moving).
Good music, sfx and juice.
Things I don't like:
Art in general. Yes, there is a ton of care put in, but the free flashgame style is a detriment to your game. It doesn't bring you in. If a random anon saw a screen of your game on steam, he would skip it in a second. Ihmo you need to change it to something else, something cool or cute.
Screen space is a fucked up for more than one reason:
In the beginning I thought turrets were unfair, but then I realized that your gameplay is not set for your screen space but for your shift pan. Personally I think it's a bad move, but of course everything can work; just think of hotline miami.
With so little screen estate in the long run can become tedious to play(claustrophobia, shift to see anything) and design (see mega man 1-6 vs 7)
Moreover a view so zoomed in makes your game feels simple (aka retro in a bad way). I could understand a view so zoomed in because you want people to focus on your art.
In the end I love the game, dunno how much you are willing to change your entire art style(it's hard), but at least consider a 50% zoom out.