Definitely needs some element of defensive attackers; what the heck are they doing inside the keep while I'm out here defending them? Sure, there's an 'Arrow Storm' sort of attack, but I'd rather they were firing those arrows with a little more discrimination. Perhaps as you upgrade the walls you add some archers who will actively help you defend against enemies at the gates?
If I were this guy's manager, I'd have to fire him.
"Sure, you've got 150 turtle shells bouncing around, but look at all the ones walking past you! Laziness will not be tolerated!"
I would suggest making all zombies in a 2 screen radius who aren't actively chasing someone respond to gunfire. At least a 5 second walk in the direction of the gunshot. I've had a single police officer survive the initial wave of zombies and pick off the stagglers as he comes across them for the last 1,136 minutes. Zombie total has gone from 11,230 to 3364 with only him left in the city.
 "Battle for Wayland Keep"){kind=icon}
 "Epic Combo!"){kind=icon}
 "Juno Spore"){kind=icon}
 "Assault Fleet"){kind=icon}
 "Zombie Outbreak Simulator"){kind=icon}
 "The Next Floor"){kind=icon}
 "Seed of Destruction"){kind=icon}