Dang!
Very good, slightly too difficult. Should funnel them a little, and perhaps a delay before they just sap your HP like it's nothing. Level Ups should give more points, highest I've gotten was level 20, and felt like the round I died on wasn't half over. Something to tell how many zombies left in that round would be very useful.
Overall very good, oldstyle like graphics are funny, and it's very well done. Love the multiple voices, now just need multiple characters :P
It'd take me a while to get all the achievs, perhaps some in-game rewards for getting achievs? They just seem like a "woohoo nice job!" without any kind of extra bonus, which'd be really nice (especially considering how many there are)
PS: Are the zombies supposed to get really riggedly fast if they go straight? They can accelerate and seem to have no max speed, not exactly fair when you're getting them in circles and wanna go the other way around, they accel right into you -.-