Crusaders are devout and dangerous medieval fighters with a mission. Crusaders also have a special ability that allows them to also build fortresses.
The Knights Templar Great Wonder, unless rendered obsolete, automatically produces a Crusader every five turns.
Years of fighting in the Holy Land led to the development of a new class of Christian knight -- one accustomed to harsh conditions and constant battle against opponents using unfamiliar and "un-Christian" tactics. Crusader knights fought both mounted and on foot, and brought their experience and learned ruthlessness and aggression back to Europe when the Crusades ended -- as in the case of the Teutonic Knights, who offered to defend Poland from invaders and ended up taking charge of a big chunk of the kingdom they promised to defend. Often originating in apparently benign orders (such as the Hospitallers, who, true to their name, ran hospitals all over Europe), crusaders were often motivated by ambition and the desire to rule their own kingdoms in the Near East.