So I built a quick library to house a few dead npcs, and it has three floors. The bottom floor is now occupied, althought the NPC is still lounging about at spawn. However, the top two floors are "not valid housing". They both use platforms as doors, have lighting, a table and some kind of chair. The background/ walls cover EVERY part of the room, even behind wooden beams, dressers and platforms. I have gone throught these thoroughly, but cannot find any reason why they are not houses! Which is annoying. Heres a screenshot- the second and third floors are not valid. Thanks!