In March 1253, Hülegü's commander Kitbuqa, who was commanding the advance guard, crossed Oxus (Amu Darya) with 12,000 men (one tümen plus two mingghans under Köke Ilgei). In April 1253, he captured several Nizari fortresses in Quhistan and killed their inhabitants, and in May he attacked Qumis and laid siege to Gerdkuh with 5,000 men and build walls and siege works around it. Kitbuqa left an army under amir Büri to besiege Gerdkuh.

In December 1253, Girdkuh's garrison sallied at night and killed 100 (or several hundred) Mongols, including Büri. In the summer of 1254, an outbreak of cholera in Gerdkuh weakened the garrison's resistance. However, unlike Lambsar, Gerdkuh survived the epidemic and was saved by the arrival of reinforcements from Ala al-Din Muhammad in Alamut.

As Hülegü's main army was advancing in Iran, Khurshah ordered Gerdkuh and fortresses of Quhistan to surrender. The Nizari chief in Gerdkuh, Qadi Tajuddin Mardanshah, surrendered, but the garrison continued to resist. In 1256, Maymun-Diz and Alamut surrendered and were destroyed by the Mongols, resulting in the official disestablishment of the Nizari Ismaili state.