This building was a former prison until the cave in. You see, prisons aren't built very carefully, they just try to get them done fast enough to stuff people in there. So while you can tell this was once a building, it definitely isn't a prison anymore, since the whole thing caved in.
You keep picturing the cave in and you see Genghis Khan (かん) climbing over the rubble. He's the one who took down this prison! And he's coming for your town next!