One of the worst things about that world is you could never get any true rest.
If I were set up someplace like that prison, i'd have booby traps and I'd kill the "outside the fence" zombies for exercise. As easy as they seem to die, I'd spread folks out along the fence line so the zombies don't mass up too much and just go to town killing them, and save the ammo for emergencies. With as many survivors as there are at the prison, they could probably make short work of those zombies hanging outside the fence.
Oddly enough, this is now a part of the plot and was mentioned in last nights season premier. Seems everyone has to take turns killing at the fences using re-usable weapons - knives, sharp sticks - I think one chick was using a medical supply walking cane that had been sharpened.