Probably just picking on the set design folks, but there's a lot of security on the door and not much thought given to the holes in the walls of the barn. We see broken and possibly rotting wood, to the point where it seemed like the walkers could have probably made a hole and crawled out. If they have enough strength to tear pieces out of a person, it looks like they'd have enough to escape their rustic wooden prison. Maybe those spots are just good places to insert chickens.