To any DC area members (this means you, Emily) (anyone else?), or anyone who is going to visit DC this summer (e.g. Brickfest), there's a great display at the National Geographic Museum. Here's how they describe it:
Experience life in the year 1271 through models of the crusader castle Krak des Chevaliers and the Middle Eastern bazaar at Aleppo. Detailed scale models peopled by more than 2,000 figurines show the Knights Hospitaller under siege in the castle and the bustling Aleppo marketplace at the crossroads of Islamic and Christian trading routes and cultures.
There's a huge scale model of a castle under siege that fills most of a room and lots of photos and descriptions of castle architecture. The National Geographic headquarters is about three blocks north of the White House (and a block from my wife's workplace). The museum is free and only takes a few minutes to walk through if you don't stop to stare at every detail of the model castle.