In 1090, Hasan-i Sabbah acquired the castle of Alamut situated in a remote and mountainous area of the Rudbar district in northern Persia (Iran). Over the next 150 years, the Persian Ismailis acquired more than 200 large and small fortresses in Iran and Syria with settlements in surrounding towns and villages, thus establishing their own autonomous states in these regions. The Nizari state survived until 1256 when the last Persian fortress was surrendered to the Mongols. The Syrian fortresses were able to resist surrender until 1273.
According to modern scholarship, Ismaili castle design in the Alamut period (1090-1256) included water engineering of the highest order. Compared to the designs of the Crusaders, the Ismaili approach to castle architecture was more elaborate: for example, water was stored for the garrison and the local population (who took refuge inside the fortress walls). Wherever the slope of a fortified hill was large enough, a well-constructed water catchment area was installed. At Saru, not far from Damghan (both in Iran), in addition to the water catchment area inside the main castle, water had also been channelled from a smaller castle a mile away. Another hallmark of the Ismaili castles was that they built at the tops of great mountains, dividing the fortifications into self-contained sections, culminating in the great citadel. These castles and fortresses were built for refuge for Ismailis who were fleeing persecution in the Middle Ages.
Read about the castles of the Alamut period in Eagle’s Nest: Ismaili Castles of Iran and Syria
on the website of The Institute of Ismaili Studies