Fantastic artwork. You've done a really good job working within the limitations and still making the game look great. The zombie designs are full of character and the use of colour in the game sells the dark, dreadful atmosphere you were going for.
I'm glad you included directional audio as its definitely a must for a game like this. I think the audio was lacking a little in other departments though. The lack of ambient audio makes the game less spooky, and the gunshot sound is a little too flat and reverby to be satisfying.
The gameplay itself is fine - just rotate and shoot. It can become fun to manage all of the zombies appearing and can quickly become hectic (in a good way). It would've been cool to see some events that could randomly happen or something to keep the gameplay more engaging.
Anyway, cool game and good work :)