Nice job, I like the idea of mixing Doom with stealth. I really enjoy the graphics. Nice lighting, good sprites, and the audio is nice too. I found a couple things I think you could address to improve it even more.
- Kenneth already mentioned the spinning boxes in the first stage. I couldn't tell what was happening when I hopped on em, it just kept turning me around every time I tried to step forward.
- There was a gate that asked for a keycard in I think the second level, but I just hopped over it?
- The first time I got on the elevator, I didn't realize what it was, so I ended up getting off and it left without me and didn't come back down. Maybe have a button you press to call/raise the elevator? It could also be quicker.
- The guards / zombies were way too weak. When you keep picking up armor, it's near impossible to die, and even when they were pressing against me, they only occasionally did damage. Their AI could probably be improved a bit too, because they tended to ignore me or lose interest even if they saw me.
- I couldn't see the bullets the enemies were shooting at first so I was confused as to how I was taking damage. Maybe make them a bright red, orange or yellow so they stand out against the darkness?
- The area where it goes grayscale confused me. Was something happening?
Another thought I had was right now, I personally find that there's a bit of a disconnect between the stealth and the "Kill em all" messages. Maybe push the stealth a bit more and make the enemies way more dangerous, so that if you're noticed, it gets really frantic and scary so you're encouraged to avoid combat? Right now there isn't really an incentive to avoid enemies.
Overall great job! The level design is really cool and I hope you keep working on it. I'd love to see what you could do with it with even more time.