Gerdkuh was a castle of the Nizari Isma'ili state located near Damghan in the region of Qumis (modern-day Semnan Province of Iran).

The word Girdkūh[1] (Persian: گردکوه) is a compound of gird (گرد) for "circular" (round) and kūh (کوه) for "mountain".

In the Chinese work History of Yuan, Gerdkuh is recorded multiple times, as Yü-r-gu, K‘i-du-bu, K‘i-du-bu-gu, and Gir-r-du-k‘ie, on top of the mount Yen-han, west of Tan-han (Damghan).

[5] The steepness of the rock and its height has been noted in the work History of Yuan, which claims no arrows or mangonel stones could reach it.

Their purpose may include the protection of the cultivated fertile ground on the castle's foot, and tax-collecting bases for passing caravans, as well as a more conventional and accessible place of storage relative to the buildings at the hilltop.

After a heavy rainfall the agricultural area on the south-east slope inside the outer defense walls is very clear ... To the west stretches the road to Semnan and the chain of the Alborz mountains.

The main inhabited area, which is visible from the plain, was a complex of two rows of buildings on the south-eastern slope, all of them at least two or three stories high.

The main water catchment area is, however, the three cisterns on the southern side of the hilltop where the ground drops steeply about 60.5 metres (198 ft).

They feature an outer wall of 3 metres (9.8 ft) in thickness made of stone and mud-brick covered by plaster, with vaulted roofs and turrets.

[6] Gerdkuh was refortified and transferred into the Nizari Isma'ili possession in 1100 by Ra'is Mu'ayyad al-Din Muzaffar ibn Ahmad Mustawfi (رئیس مؤید الدین مظفر بن احمد مستوفی), a secret Isma'ili convert and lieutenant of the Seljuq emir Amirdad Habashi (امیرداد حبشی), who in turn had acquired Gedrkuh in 1096 from Sultan Barkiyaruq.

However, unlike Lambsar, Gerdkuh survived the epidemic and was saved by the arrival of reinforcements from Ala al-Din Muhammad in Alamut.

As his position became intolerable, he asked Hülegü to be allowed to go meet Möngke in Mongolia to persuade the remaining Ismaili fortresses to surrender.

Möngke rebuked him due to his failure to hand over Lambsar and Gerdkuh, and ordered a general massacre of all Nizari Ismailis, including Khurshah.

[10][15] The Mongols had built permanent buildings, houses, and defensive walls near Gerdkuh, the ruins of which still remain today in Hajjiabad-e Razveh and other nearby villages.

[6] The Qajar king Naser al-Din Shah (1848–1896) encouraged Shaykh Mohammad Mehdi Abdol-Rabb-Abadi to investigate the site, whose brief report contains accurate measurements.

[5] The site was visited multiple times by Peter Wiley, who left a detailed description in his book Eagle's Nest.