I worked on the back panel and upper half of the house recently. I built the sidewalls and front front 1/2" foamboard cut to the sizes i want, and used 3/16" foamboard to make a complete back panel for the house. The back panel has windows, as you can see in what will be each room. The windows that line up with the staircase area or hallway will be stained glass or something similar on the back of the house. The back is fully detachable with velcro dots holding it on.

that is pretty much all there is to explain for the last two days worth of work. the interior walls will also be made of foamcore when i get to them, but that will definitely be a while....