The front window walls were built up with thicknesses of foam core to box in the sides of the windows. Then they are decorated with side and top panels cut out of heavy card. My favorite substitute for bristol board is cereal boxes.
Wood trim and wallpaper finish off the wall beautifully.
The inside wall of the kitchen gets a wood strip treatment by the doorway and a panel and wallpaper treatment toward the front of the house.
The outside wall with the back door is a very interesting wall. It houses the sink and appliances in little divided cubbies and it hides all the wiring from the other levels behind the walls. The ceiling has beams going across it in two directions which also gives it more interest than a plain ceiling. I still have to fill in the edges and touch up some paint on those.
I'll probably go ahead and make the sink but save the stove and fridge for another day. What! No microwave? I guess they really did have it rough back then. :)