According to Hamdollah Mostowfi, the fortress was built in 840 at an elevation of 2,100 m. It was built in a way that had only one passable artificial entrance that wound its way around the cliff face (the one natural approach, a steep gravel slope, was too dangerous to use); thus making conquering the fortress extremely difficult. The fort had an unusual system of water supply. The top was extremely narrow and long — perhaps 400 meters long, and no more than 30 meters wide in any place, and usually less.