I got a little lost I think. I didn't make it past level 1. I really like the idea, and think it could make an interesting game, but, there was some tricky things going on. I didn't have time to read all the obstacles on the first loading screen, and it definitely is a little tricky jumping and shooting backwards. I did like the walk animation, though. I also had a weird thing happen that might be tied to the wave spawn that you mention. It happened to me twice where a verticle line of zombies spawned on top of me. I also didn't die right away after reaching zero health. Interesting concept tho, nice job.