Great game!
The gameplay is straightforward and the control is easy. I like the concept which could be a solid foundation for a story based game or for additional game modes.
I find it a bit to easy though. After getting accustomed to the controls the zombies died like flies. Difference between the toughness, speed and power of the zombies would force the player to tactics.
It would be great to have a survival mode where the zombies come in waves with different zombie types.
Nevertheless keep up the good work!