The clue in this post is that he can climb the stairs. Those are skeleton buildings and if looked at with your hammer enabled you will see they are about the same strength as cheezie cloth. On the other hand grey dwarf buildings are pretty sound to start with. They are good to use as shelter while roaming, exploring as you can fit a portal in them, put your workbench up on the little platform, destroy their chest and put in your own but they are not troll proof. Will take quite a few hits though. What they are good for is LoS and annoying trolls though I have just hunkered down in them and the troll finally wandered away. Skele buildings are worthless for shelter unless you carry mats for stonecutter with you on your travels.