The construction of the existing stone walls of the Derbent Fortress is dated to the 6th century AD. This dating is based on Arab written sources from the 9th-10th centuries. Arab authors associate the founding of the city of Derbent with the name of the king of Sasanian Persia, Khosrow I Anushirvan (531–579 AD). Furthermore, the reports of Arab authors are supported by Pahlavi construction inscriptions located on the northern wall. Similar inscriptions are absent on the southern city wall. This fact indicates that the southern and northern walls were built at different times. At the same time, the similarity in masonry technique leads to the conclusion that the construction of the southern wall could not have taken place much later than the northern one. It is likely that the construction of the city walls was carried out in two stages, separated by a short period of time. Moreover, the northern wall was built earlier, as it was intended to protect the Persian population from raids by northern nomadic tribes. The construction of the southern city wall dates to the 6th century. The length of the southern city wall is about 3 km. It had seventeen towers, all rectangular in shape, spaced 180 meters apart. This fact is explained by the fact that the southern wall did not serve a serious defensive function. Its purpose was to serve as a protective enclosure for the customs zone, which determined its construction features: simplicity and economy of the line, thinner wall thickness, armor masonry that resembles facing more than an anti-ram armor, and so on. The southern city wall is built from large blocks which, laid in stretcher and header courses, form the outer facing of the masonry, while the core of the wall is filled with rubble stone bonded with lime mortar. Unlike the northern wall, it has no bonding with the citadel walls and only adjoins tightly to its southeastern corner. Also, unlike the northern wall, the southern wall has no foundation and rests on a well-leveled surface. The southern wall includes gates: "Kala-Kapy," "Bayat-Kapy," "Orta-Kapy," and "Dubary-Kapy."

Source: 

https://derbentmuseum.ru/monuments/steny-derbentskoj-kreposti-yuzhnaya-stena/