Sweet game. I really like how the horde mechanics felt. They followed the player in a nice swarm.
The general concept is good. Very curious where you could have taken this if you had more time. The pixel art was great. I like that the zombies looked like they were dragging one of their legs.
Outside of the obvious lack of sound, I'd think it'd be nice to be able to hide from gunfire behind cars and fences. That would open up some strategies. Maybe some variety in zombies and enemies? Zombifying some tougher enemies create special zombies?
Overall, great job!