Pretty good.
I liked it quite a bit. It could've been a little better, by adding the ability to set barricades, or pick up zombies/humans in mid-game and place them somewhere. But, with the small frame of time you made it in, it's of good quality. The music gets a bit repetative, though.
The things that the humans say made me laugh. "LOL ZOMBIES" and "GG NUB". Damn.