In Derbent, in the northeastern part of the Naryn-Kala fortress, there is a rectangular reservoir. Not found in the territory of the European part of Russia, this type of architectural structure became extremely widespread in the hot arid regions of the country. The absence of above-ground, nearby water sources forced the creation of an artificial water supply system for the city, which amazed travelers with its grand scale. Water from springs located high in the mountains flowed by gravity through underground stone pipes, first entering the fortress, then the upper and lower city. Water-collecting fountains were arranged along the water’s path.

A large rectangular underground reservoir served as the main water storage for the defenders of the citadel. Water came through underground stone channels from mountain springs. The rectangular cistern was adjacent to the service yard of the khan’s palace. This vaulted structure is positioned so that its western end is completely embedded in the ground, while the eastern end is only half embedded. Here, the reservoir has an end wall with a passage. The cistern is covered by a pointed vault with an eight-meter span and a thickness of thirty-five centimeters, reinforced by two pointed arches. The length of the cistern is about twenty meters, and the height is about nine meters. At the apex of the vault, there are two square openings. A sloping passage leads from the surface of the ground to the end wall built into the slope, ending with a low (about 60 centimeters) arched opening located more than three meters above the bottom of the cistern. This channel possibly served to fill the cistern with water. After Derbent was taken by Russian troops, the rectangular reservoir, like many structures in the Naryn-Kala citadel, underwent some changes. The fortress garrison began using the reservoir as a warehouse. A passage was cut into the end wall, into which iron gates were installed. A metal ladder leading to the bottom of the reservoir also appeared.

Source:

https://derbentmuseum.ru/monuments/vodohranilishhe-pryamougolnoe/