Krak de Chevaliers (a name coined as recently as the 19th Century when romanticism caused a revival in interest in the crusader ruins) has over time been the residence of Kurds, the County of Tripoli (which lay just north of the Kingdom of Jerusalem), the Knights Hospitalier, and was conquered by the Mamluk Sultan Baibars after a 36 day seige. It watches over the strategic Homs gap, and is only one of a series of castles that helped Christian crusaders keep control of the Holy Land.
As one of the most important preserved medieval castles in existence, it holds many fascinations including a large chapel, hall of the knights, inner and outer wards, many walls including decorative frescoes. If you're lucky you'll find your way into a dark cavern down a seemingly never-ending stairwell -- bring a torch!
From the Lonely Planet: Described by TE Lawrence as the ‘finest castle in the world’, this hilltop Crusader fortress might be 800 years old but, like a good botox treatment, stands tight and taut against the ravages of time. It’s the classic blueprint of a medieval castle, its thick outer walls separated from the inner structure by a moat dug out of the rock. Inside, it’s a minitown, complete with a chapel, baths, a great hall and a Gothic loggia. The most visible sign of ageing is the vegetation that grows from its walls; nothing a good shave wouldn’t fix.