The prison seems so obvious. And you would think Rick would know where all the prisons in the area were. On talking dead the producer said no one had gotten to it yet because of the herds of zombies blocking thier way. But I was thinking they wouldn't have been ready to take it the last time we saw them anyways. I can't imagine them being enough of a team to be able to take it or imagine them being desparate enough. They've given up any hope now of any kind of normal existence.