[Raellus]

Great resources, gents. Thanks for sharing.

Osprey Publishing's Fortress series has some interesting volumes that are germane to T2k. I can speak to a couple specific examples.

Fortress 11: Crusader Castles of the Teutonic Knights (1) AD provides a great overview of castles in Poland, many of which are located on the banks of the Vistula. I used it to build encounters for a Pirates of the Vistula PbP campaign, and to write the Rook's Gambit adventure module (link in my sig).

Special Forces Camps in Vietnam 1961–70 (Fortress) is a good resource for GMs wishing to include fortifications that one might expect to encounter around cantonments of around company to battalion size.

Other titles cover some of the fortifications previously mentioned by other posters in this thread.