Israel offers several sites with ruins of Crusaders' fortresses (about a dozen of them including Aqua Bella, Chateau Pelerin, Yehiam). I have been to Montfort, Belvoir and Apolonia and there is not too much left of any of them to be honest. Monfort makes a nice trip but mostly for its location. Belvoir still clearly shows its massive star-shaped fortifications resembling later constructions by Vauban. It also offers its visitors a nice panorama if you happen to visit Israel in spring. The Crusador architecture also features in the WHS of Acre and the TWHS of Caesarea.