I've gone for 25mm high walls. This allows a 20mm high door, plus 10mm square windows, set 10mm above ground level, leaving a 5mm strip along the top to keep it all together.
This means I get a nice butt-joint on each corner, giving stronger joints and smoother walls - no exposed lines of foam at the edges which will show through the texturing/painting.
I'm giving them all a flat roof for now. I've gone for 1, 2 and 3 storey structures. Some have chopped-up matchstick ladders to provide roof access.
Others obviously have internal stairs to the first floor, then a ladder to the top.
Each storey is separate, allowing me to position people inside and forget about them, thus losing the game because I can't keep track of where everyone is.
Hmm... hang on, I may have to rethink this last part of the plan!