So tower falls on a reinforced fence. I would have expected that between the remains of the fence and the rubble from the tower it would have been pretty difficult to get through. Given how uncoordinated the walkers are and their penchant for getting kebob-ed on any pieces of wood/metal sticking out, I would have expected quite a few of them to get stuck while trying to make their way through the rubble, which would make it even more difficult for the rest to follow. So, in summary, there really shouldn't have been more than a few of the more agile of the herd inside of the compound. The rest should be either stuck after skewering themselves on some broken planks of wood or fence, or simply not being able to squeeze through the wreckage.