|Castles on the Web has great information; check here first!
Castles of Britain is a comprehensive educational site you'll want to visit!
Medievality has so much info about castles and the Middle Ages, it is mind boggling!
This PBS page has great information, and many links to more (on the bottom of the page)
Medieval castle.com has more about castles than you might ever need to know!
Take a virtual tour of Raglan Castle