It was actually pretty cool!
I was worried about the projectiles disappearing and not having enough to get rid of the zombies but there was just what we needed.
I did try to kinda speed run the game and when I had all of the zombies following me in the last room I decimated them! ^^ And yes I tried going in the acid trap even though it seemed obvious (seeing the depth of it) there was no way to go back) but I think you should have done something with the doors opening: Since you can technically speed run the game easily, maybe opening the door when all the enemies in the room die would've avoid what I did.
Since you are on Unity you can do that by: Having a game object representing the group of enemies and send an event to check if at least one enemy (children of this game object) remains in the group. (the parent game object) There are other ways to do it of course and maybe it's obvious to you but I'm throwing it there if it can help.
[SPOILER ALERT!]
I have found the end of the level pretty obvious since I thought about rigid bodies but I have no idea if it would be obvious for someone who didn't use a game engine (aka a "normal" player) without any help.
[END OF SPOILER ALERT]
I wrote a lot to explain only 2 minor issues really but it didn't break the fun at all in my experience! :)
Other than that the game was very easy to pickup if you played FPSs before, the AI was simple (zombies! Did you use a navmesh by the way?) and it was working correctly, the theme was well used and the light effect on the projectiles added a nice little touch to this kind of dark world, the zombies were funny to look at which also added some personality to the game and the end added a mini puzzle (survival fps horror + puzzle? :P ) to use your brain before the one you are supposed to retrieve.
So in the end, I had a lot of fun playing it!