http://www.spartacus.schoolnet.co.uk/Normans.htm
A large resource with lots of cross references to detail such as a glossary of castles, an example of the feudal system, information about medieval weaponry and the full background history of the Normans.

http://www.castlewales.com/home.html
Good photographs of the most famous castles in Wales and more information about Welsh castles and Welsh medieval history.

http://www.castles-of-britain.com/castle6.htm
Contains a research page with lots of links to different aspects of castles and their history.