I am sitting in the shadow of what is probably the most awesome single castle ever built: Krak des Chevaliers, in what is now Syria.

Located in the mountains to the northwest, Krak des Chevaliers controlled the Homs Gap and thus, the flow of traffic between the Mediterranean and the inland cities, as well as land trade between Asia Minor and the Holy Land. For a century and a half, the Knights of the Hospital of St. John occupied this massive fortress, and it withstood at least 12 attempts to take it.

The walls were literally dozens of feet thick in places, and the main passageway into the castle went up steeply, built in a zigzag pattern that would force the enemy right into the teeth of the waiting defenders, and then, they had to face a set of only slightly smaller inner walls. Inside, they could store enough food to feed a large army for more than a year. It finally feel in 1271 to Sultan Baybars and the Egyptian Army, when it was undermanned with no hope of reinforcements. Even then, the Sultan had to forge a letter from the Order's Grand Master before the remaining knights would surrender.

Here's a view from farther away, so you can get a little better picture of the place.

Detail : Krak des Chevaliers - A bunch of pictures showing the interior and exterior of the castle.

The Knights Hospitaller - Here is a description and history of the feudal order who built and maintained Krak.