I strongly endorse "Strongholds and Dynasties" by Mongoose (MGP8801). I own it and have found it to be remarkable. It is made of 3.0 but works just as well with 3.5. Theres 128 pages on strongholds, including construction, rooms, and bunches of extras light dragon-powered steam engines. The other half of the book has mass-combat rules, Empire Rules, and Provience Management stuff (cool if you are into that sort of thing). I highly recomend it.