The crusaders stole a lot of muslim science heritage from the muslims during the crusades.!
A german tv channel makes this very clear, but it's in german language if u understand it:
Das geheime Wissen des Islam
With Arabic undertitle:
علوم الإسلام الدفينة - الجزء الاول
http://www.youtube.com/watch?v=jD0HEFtCKRA