Neat game. I like the sprite art, it's simple but pretty effective. I also thought it was cool that the fake and real zombies had different death animations, nice touch. Like someone below already said, the level of polish is pretty high with the alerts that the disappearing bar is full, ammo upgraded, etc. Very clear with sfx and everything. Also made pretty clear that ammo = health with the sprite work on the health bar. Well done.
If you wrote the music (which was cool), you could ramp up tension between waves by adding elements over time. Same composition and all, but add some percussive elements later on, maybe new melody, etc.