The crusades mainly took place on what is nowadays Jordanian territory. When the Arabs attacked the holy places of the Byzantines, they called the crusaders to their aid.
The crusader fortress of Karak is built very strategically on top of a mountain along the King's Highway. It was the battle field where Saladin and Richard Lionheart conquered each other. If only to admire the view the fortress is worth a visit.
For the greater part the castle was built underground. You may dwell through dark underground corridors and take a look at the soldiers' rooms and knights' halls.