Nice job! The art is good, but some more color would be nice. I'm sure you chose to use only black on purpose, but throwing in a few shades of grey here and there would have been nice. Can't wait to see your next project.
I don't think it's that the shooting is *broken*, it's just that there's no indication that you're actually shooting. Once I realized that shooting made no sound or graphic of any kind, I was able to kill a handful. I also couldn't really tell if a shot was a hit or a miss, except for when things died. Just work on making the game more responsive to the user's input.