The castle was originally established by a local lord who in 1132-3 sold the fortress of Qadmus to the Ismaelis from where they began to establish themselves in the area. His son later sold al-Kahf to the sect, as part of their programme of rapid acquisition of eight castles in the area from 1132 to 1140.
From 1164 to the early 1190s, the Ismaeli leader, Rashid al-Din Sinan (known to the Crusaders as the old man of the mountain),operated from this remote mountain fastness. The castle is mentioned in the historical records of the 12th-century Crusaders in connection with exchanges between the sect and the Knights Templar with whom they shared an unspoken entente. In 1197, Henry of Champagne, Regent of Jerusalem, sought an alliance with Sinan's successor to counter Muslim pressure on the fragmented Crusader state.
He was invited to al-Kahf where, to demonstrate the fanatical devotion of his followers, the Assassin leader bid two of them to hurl themselves from the castle parapets, which they did without hesitation.
This ambiguous relationship with the Crusaders continued the next century, particularly under St Louis of Acre, the French king who led the disastrous Sixth Crusade against Egypt in 1249.The Ismaeli leader set up an assassination attempt on St Louis but his agents were detected and sent back to al-Kahf with gifts to illustrate Louis' magnanimity later Louis sent an envoy, Yves le Breton, to al-Kahf bearing presents, as a result of which an alliance was concluded. Yves was the first Western visitor to the Ismaelis to take an interest in their doctrines.
Not surprisingly, al-Kahf was the last of the Ismaeli strongpoints to fall to centralized Arab control. It was not until 1273 (two years after his successful siege of the Krak) that Baibars captured the castle as part of his final elimination of the Ismaeli presence. The castle remained a military post into Ottoman times, probably serving as a usefully remote place of detention. In 1816,the British resident of Lebanon, Lady Hester Stanhope, took up the cause of a French captain who had been taken captive and held in the castle .He was rescued at her behest by the Ottoman governor of Tripoli, who also carried out her wish that the castle be razed