The valley of Alamut lies in the mountainous north of Iran, between the Siah Lan mountain range and the Alburz range to the south, some thirty-five kilometers northeast of Qazvin. The river Shah Rud runs through its center. Toward the end of the eleventh/fifth century AH, the valley became a haven for an Isma'ili state that remained independent from surrounding medieval polities, protected by the difficult terrain that surrounded the area (1090-1256/483-654 AH). The valley's rocky slopes are home to a number of medieval fortifications. One of these, itself named Alamut, served as a headquarters for the aforementioned Isma'ili state and its ruins are preserved today as an archaeological and tourist site. It is thought to have been founded in the early Islamic period, and reinforced during the period of Isma'ili occupation. Alamut and the Isma'ili state fell in 1256/654 AH. During the reign of the Safavid Dynasty (1501-1722/907-1135 AH), the abandoned fortress was converted to a prison.
Hourcade, B. "ALAMŪT." Encyclopædia Iranica, I/8, pp. 797-801.