Krak des Chevaliers in the Lebanon has the reputation as the strongest and most dramatic of the crusader castles in the Middle East, but in my opinion Belvoir runs it a close second.
Set high on the hills overlooking the Jordan Valley and an important ford, the castle was a textbook example of concentric fortification. Built of the local black basalt, it was surrounded by a dry moat 90' deep and wide and defended by a series of salley ports cunningly concealed in recesses in the walls.
Its strength was shown when, after the Battle of Hattin, a small garrison hopelessly defied Saladin and kept his forces at bay for eighteen months. It only fell when Saladin discovered its weak point.