Update your link in first post, its not working
You should make GUI more responsive (buttons doesn't look like buttons)
Nice idea but its too fast at start, imo you should add more kind of zombies, including some bosses, make regular zombies easier so their function would be only making profit, so player has time to rise his defense before boss arrives, and boss (not regular zombies) should bring real challenge to the player.
Such approach may hook player better inside your game, current gameplay isn't really entertining