castle is from Kallistra, yes.
the roofs are made of thin sheets of wood (not balsa, but balsa'd do fine too I presume), while the "brick" walls were made from modeling clay - pressed flat, left to dry, and then carved the stonework with a tool. Found a tutorial for it on the Flames of War website (
http://www.flamesofwar.com/Default.aspx?tabid=110&art_id=708&kb_cat_id=24 - was done by casting the wall but it's more or less the same).
btw. here are some more buildings
these walls were made from something I don't know the English expression of - basically it's about 5mm thick, consists of foam with a thin layer of card on each side.
The wooden framework was made of wood, and the non-wood roof from old brush bristles.
cheers,
M