Just a tip - if you have stairs inside a house it's making the area above them one tile higher than the area below them. This means that for the ceiling to be on the same level (that is, on top of the walls above and below the stairs) the bottom level needs to have the walls one tile bigger. So in this case, the walls below the stairs would be three tiles high if the ones on top are two tiles high (or the top ones would be one tile high instead of two).