Pretty good prototype
I liked the gameplay; I thought the turn-based aspect fit the game dynamics really well. We've all seen hundreds of zombie shooters where it's just a hassle to try and change weapons to meet the particular demands of the level, this was obviously different. The sfx were good, thought I didn't like the choice of music for this type of game. I thought the amount of movement varying per weapon was a little unusual, especially since it didn't seem to correlate with weapon weight (heavy weapons = less movement), but I thought that concept worked well with the strategy aspect of the game.
I'm not going to complain about the graphics re: characters and enemies, and I realize this is a stepping stone to a larger project, but you could've made the terrain a bit less bland. Also, using a pic from Zombieland for the title screen is probably setting the expectations for the graphics a bit high and resulting in so much negative feedback. I would also recommend making the level names a bit more descriptive since you've stated that each level has its' own unique challenges. Adding some sort of general radar map in the viewable screen, even if it's hazy or doesn't show every approaching zombie would probably be a plus since your first X number of shots are just guesswork. This is a good jumping off point for a shooter, and I look forward to the finished product.