Lego is very good at designing a very particular Castle. The recent Lego Kings Castle (70404) is a good example. They usually contain a massive gate house, two wall towers (bastions) and usually one big tower. The design is always extremely modular, with movable walls, and reconfigurable layouts. The only problem is that Lego has made this castle many many times. Once you own one or two they start to blend together and get boring.
Durazno33 has proposed a CUUSOO project called “The King’s Castle Keep.” Which sets out to fix the lack of variety in Lego castles. A three level keep meant to compliment your Castle collection. Featuring a throne room, treasury, kitchen and dungeon. The design would be able to open up from the sides, but still look great from all angles. It even includes an obligatory catapult.
Check out all the details over on CUUSOO: http://lego.cuusoo.com/ideas/view/49537