Gerdkūh or Girdkūh (Persian: گردکوه, also known as دژ گنبدان Dezh-e Gonbadān, "fortress of the domes") was a major castle of the Nizari Ismaili state located near Damghan, Iran. Due to circular shape of mountain peak, it was named Gerdkuh, Gerd (گرد) for "circular" (round) and kuh (کوه) for "mountain". The castle was surrounded by a triple ring of forts making its position seemingly impregnable. Hasan Sabbah and other Nizari leaders often sent their families to this castle for refuge. Hulagu Khan with 5,000 men started a siege against Girdkuh in 1253. Girdkuh held out against the Mongol siege for 17 years, and was the last Nizari stronghold in Persia to surrender to Mongols.
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