Alamut (Persian: دژ الموت) was once a mountain fortress in the arid hills by the Elburz mountains, south of the Caspian Sea, close to Gazor Khan, near Qazvin, about 100 km from present-day Tehran in Iran. Only ruins remain of this fortress today.
The fortress was built in 840 at an elevation of 2,100 m on ruins of Sassanid castle. It was built in a way that had only one passable artificial entrance that wound its way around the cliff face (the one natural approach, a steep gravel slope, was too dangerous to use); thus making conquering the fortress extremely difficult. The fort had an unusual system of water supply. The top was extremely narrow and long — perhaps 400 meters long, and no more than 30 meters wide in any place, and usually less.
In 1090 the fortress was infiltrated and occupied by the powerful Ismaili sect of Shia Islam (known to the West by the nickname Hashshashin or Assassins), and was then fabled for its gardens and libraries. The ruins of 23 other fortresses remain in the vicinity.
The fortress was destroyed on December 15, 1256 by Hulagu Khan as part of the Mongol offensive on Islamic southwest Asia. The fortress itself was impregnable, but Ruknuddin Khor-shah surrendered it without a real fight, in the vain hope that Hulagu would be merciful.