Potentially awesome defender game.
This is a perfect example of what a good defender game should be. The effects are amazing for flash, the sound effects are top notch, and the concept is simple yet effective. The music really sets the ambiance and tone of the game (28 Days Later FTW). The only complaint I have is the XP needed to upgrade. I suggest lowering it to maybe 100xp per weapon upgrade and 250 XP per wall repair? It seems that I always spend all my XP on repairing the damn wall.
Maybe you could slow down the civilians and add even faster zombies or infected in later stages to make things interesting. Or maybe even add some ground troops to help you defend off the zombies and you can call in back up after a certain amount of time, like the airstrikes. Maybe even buying some new weapons? IDK, these are just suggestions.
Overall, this is a solid defense game. There are a couple of downers, but the ups of this game outweigh them and make this game awesome. Please make a V2 of this game! And I hope you make more games like this!