Yesterday, I played around a bit with sketchup and made a plan for a little castle. Its not a plan for HA, but I wanted to share it.
Its made of single pieces, you can cut from Styrodur. I am not sure about the english name, but I think you call it polysterene. In Germany, you can buy it 2 cm thick. So all of the pieces can be cut from 2 cm thick Styrodur.
Regard, that I used Inch instead of centimeters in Sketchup, but thats not the right dimension.

The sketchup-file can be found on my homepage under "Plne":
www.gidian-gelaende.de

Greetings from Germany

Gidian