Good protype,Controls is fine for me,fun to drive and kill zombies.Also i like visuals and idea itself.
But...
AI is bassicaly braindead so no coop.You play by youself or just try to move AI around so he can kill zombies))
I suggest to remove walls from towers so AI can easily charge zombies,walls pointless btw.
Maybe add ability to point AI in direction,like follow me or go to kill zombies in this spot,Maybe add more AI,like 3-4 more cars so gameplay can be more chaotic? Or add ability to like shoot chainsaw,bullets.This will help AI too,cuz it can kill zombies more easily from distance?
Feel free to use any ideas i gave you.Hope this helps,good luck!