Zombies... duh..
For starters GREAT GAME! I love it.
8 for graphix because i really liked the animated blood, but the base was a bit ugly and it stayed the same no matter how much you upgraded the base.
Sounds where great, a 9 because when you have the laser cannon and kill a row of like 10 zombies the sound of the falling zombies get really hard.
Violence 10, hmmm why would that be?
Humor 10, I think killing zombies is quiet funny.
Why is the evolution of the really big zombie a HeadCrab (isn't that supposed to be the other way around xD)
Bug and potential spoiler:
Overkill powerup + Laser Cannon, gives about 88000 xp per level, maybe you could fix this.