OK. So if you build a fence around the prison why not dig a moat around it - can zombies drown?
Digging a little deep into the thread here, but I've wondered this all along. You dig a five foot wide trench all the way around the fence, on the zombie side, using distractions to get the whole thing dug eventually. Carol even said they'd had a quiet few months before the season started. Make it 6-7 feet deep, the zombies fall in and get stuck, and you burn them up every once in a while.It's not perfect, and there'd be a dead body/detritus build-up after a while , but it's better than letting hundreds upon hundreds of zombies gang up on one area of your weak ### fence every single day.
They'd never reach the fence this way. You make up some long ### spears and stand on the strip of grass outside the fence but inside of the trench every once in a while, stabbing zombie heads at your leisure as they're stuck in the trench. Move the bodies out once in a while or burn the whole trench once in a while.
Edit: And if you want to argue that they'd never be able to finish it on the outside of the fences, then fine. Do it between the outer and inner fences, leaving wide walking paths on either side for the survivors to use the aforementioned long spears to kill trapped zombies at their leisure. Open up a section of the outer fencing, letting the zombies into the trench every few days as they build up, safely trapping and killing them all in the trench between the fences whenever you need to, with no risk to the fences and no risk to the living survivors doing the spearing. It would be so easy--like spearing zombies in a trench. Wait.