Construction of the Crusader castle began in the 1140s, under Paganus, the butler of King Fulk. The Crusaders called it Crac des Moabites. Paganus was also Lord of Oultrejordain (Transjordan), and Kerak became the centre of his power, replacing the weaker castle of Montreal to the south. Because of its position east of the Jordan River, Kerak was able to control Bedouin herders as well as the trade routes from Damascus to Egypt and Mecca. His successors, his nephew Maurice and Philip of Milly, added towers and protected the north and south sides with two deep rock-cut ditches (the southern ditch also serving as a cistern). The most notable Crusader architectural feature surviving is the north wall, into which are built immense arched halls on two levels. These were used for living quarters and stables, but also served as a fighting gallery overlooking the castle approach and for shelter against missiles from siege engines.
In 1176 Raynald of Chatillon gained possession of Kerak after marrying Stephanie of Milly, the widow of Humphrey III of Toron (and daughter-in-law of Humphrey II). From Kerak, Raynald harassed the trade caravans and even attempted an attack on Mecca itself. In 1184 Saladin besieged the castle in response to Raynald's attacks. The siege took place during the marriage of Humphrey IV of Toron and Isabella of Jerusalem, and Saladin, after some negotiations and with a chivalrous intent, agreed not to target their chamber while his siege machines attacked the rest of the castle. The siege was eventually relieved by King Baldwin IV.
After the Battle of Hattin in 1187, Saladin besieged Kerak again and finally captured it in 1189. During the siege the defenders were said to have been forced to sell women and children into slavery for food (this is also said to have happened at the siege of Montreal).
In AD 1263, the Mamluk ruler, Baybars, enlarged and built a tower on the north-west corner. In AD 1840, Ibrahim Pasha of Egypt captured the castle and destroyed much of its fortifications.
The castle extends over the southern part of the plateau. It is a notable example of Crusader architecture, a mixture of European, Byzantine, and Arab designs. Its walls are strenghthened with rectangular projecting towers, long stone vaulted galleries are lighted only by narrow slits, and a contains a deep moat from the west which completely isolates the site.
In the lower court of the castle, there is Karak Archaeological Museum, which was newly opened in 2004 after renovation work. It introduces local history and archaeology of Karak region- the land of Moab- from the prehistoric period until the Islamic era. History of the Crusader and Muslims at Karak castle and town is introduced in detail.