Castle Magic, located in the modern state of Idaho, designs and builds actual stone castles with hardwood floors, heavy timbered roofs, drawbridges, and all the medieval trimmings. Their construction technique uses mortared stone outer surfaces for authenticity, but fills the walls with reinforced concrete and a layer of Styrofoam™ insulation for energy efficiency. Electrical conduits and plumbing are run through the walls during construction.
The web site has plans, perspective drawings, and photographs of some of their design options. Prices start at about US$200K for small castles and...well, much higher...for the large ones. But their web site does say that they offer a discount to SCA members.
Thanks to Lord Arthur of Lockehaven for posting the link to Castle Magic.