As you can see -- Recluce is right, that site is quite good. You really can't appreciate the breadth of information found there without sitting down and reading for a few hours... it's got a -lot- of stuff.
I've been using that site as a reference for quite awhile now.. love it. Smart move posting it here, Recluce! - I'd never considered posting it (not for fear of revealing my secrets, of course, but just because the thought never occurred to me).
Just so my post doesn't come off as one long "atta boy" -- here are some other interesting Castle-related links:
The Knights Templar
The History of Britain
The Best Collection of Medieval Military Accounts Ever!
(The site seems to be down, I hope it isn't gone forever - it was an amazing site.)
Maps of Medieval Cities
Even with all those.. I still recommend The Medieval Sourcebook as the best all-round reference.
Forge not works of art but swords of death, for therein lies great art.
"The Gods made heavy metal and they saw that it was good." - Manowar