The South Wall Gates — Kala-kapy (Fortress Gates) (from the Arabic "kala" — fortress, citadel) are the uppermost first gates from the citadel on the southern city wall. Unlike other gates, they do not lead into the city but directly to the entrance of the citadel. Therefore, there is another name for these gates — Khan-kapy (Turkic Khan’s gates, Khan’s gates). The gate opening probably appeared when the city was under Turkish control, during the period of intensified conflict between Derbent and Shirvan. Despite some similarity to other city gates, they are clearly of later origin. Specialists date the construction of the gates to the 18th century, the period when Derbent became part of the Quba Khanate, and attribute them to Fatali Khan of Quba.

From the outside, the gate opening has a flat lintel, while from the city side it has a pointed arch shape, making them similar to the ancient city gates, which are characterized by this very design. On the eastern side of the gates stands a semicircular watchtower made of rusticated (roughly hewn, with a convex surface) stone.

The technique of constructing a bastion with a rounded projection housing a guardhouse is undoubtedly closer to the Turks, who were well acquainted with European fortification, rather than to the Persians, whose fortifications were much more primitive. This technique also suggests that next to this bastion and under its protection, between it and the citadel wall, there must have been an opening in the wall for secret sorties under the fortress wall. Later, in the second half of the 18th century, the Kala-kapy gates were built in place of this opening. Worth noting is the reconstructed and heightened section of the wall near the guardhouse — it was meant to serve as a kind of protruding bastion at the fortress and during prolonged sieges to protect against flanking fire. This entire reconstructed defensive complex is laid with careful yet decorative masonry of rusticated blocks, used for the first time in Derbent, indicating that a visiting engineering and construction team worked here, possessing skills in both civil or even palace architecture and Western Russian fortification and engineering arts.

In 1920, the gates collapsed, and for a long time a breach existed in the wall at their place. In 1960, they were restored. The outer facade of Kala-kapy is extremely simple — a rectangular opening with a wedge-shaped lintel in the fortress wall; such gates are also shown in one of the early 19th-century drawings. The inner facade of the gates is more interesting; during restoration, by analogy with earlier gates, the vaulted passage ceiling was given a pointed arch shape.

Sources:

https://welcomedagestan.ru/placepost/kala-kapy/

https://pro-derbent.ru/kala-kapy-xvi-xviii-veka-vorota-yuzhnoj-steny