Gates of Alexander
The Gates of Alexander was a legendary barrier supposedly built by Alexander the Great in the Caucasus to keep the uncivilized barbarians of the north (typically associated with Gog and Magog) from invading the land to the south. The gates were a popular subject in medieval travel literature, starting with the Alexander Romance in a version from perhaps the 6th century. The wall has been frequently identified with the Caspian Gates of Derbent, Russia (see below) and with the Pass of Dariel or Darial.
An alternative theory links it to the so-called "Alexander's Wall" (the Great Wall of Gorgan) on the south-eastern shore of the Caspian Sea, 180 km of which is still preserved today, albeit in a very poor state of repair.
In reality, both structures were built by the Persian monarchs. Derbent (in Persian دربند Darband, meaning "closed gates"), was established in the end of the 5th or the beginning of the 6th century, when the city was refounded by Kavadh I of the Sassanid dynasty of Persia. The Great Wall of Gorgan was built during the Parthian dynasty simultaneously with the construction of the Great Wall of China and it was restored during the Sassanid era (3-7th century)
Literary background 
The name Caspian Gates originally applied to the narrow region at the southeast corner of the Caspian Sea, through which Alexander actually marched in the pursuit of Bessus, although he did not stop to fortify it. It was transferred to the passes through the Caucasus, on the other side of the Caspian, by the more fanciful historians of Alexander.
In the Alexander Romance, Alexander chases his enemies to a pass between two peaks in the Caucasus known as the "Breasts of the World". He decides to imprison the "unclean nations" of the north, which include Gog and Magog, behind a huge wall of steel or adamantine. With the aid of God, Alexander and his men close the narrow pass, keeping the uncivilized Gog and Magog from pillaging the peaceful southern lands. The nature of the pass is never very clear; some sources say it is a pass between mountains, while others say it is a pass between the peaks and the Caspian Sea.
The story later appears in the Qur'an, Surat al-Kahf 83-98. Here, Alexander is called Dhul Qarnayn, a savior figure who builds a huge iron gate to defend people at the foot of two mountains from Gog and Magog. However, Muslim scholars are not unanimous in their identification of Dhul Qarnayn as Alexander.
During the Middle Ages, the Gates of Alexander story was included in travel literature such as the Travels of Marco Polo and the Travels of Sir John Mandeville. The identities of the nations trapped behind the wall are not always consistent, however; Mandeville claims Gog and Magog are really the Ten Lost Tribes of Israel, who will emerge from their prison during the End Times and unite with their fellow Jews to attack the Christians. Polo speaks of Alexander's Iron Gates, but says the Comanians are the ones trapped behind it. He does mention Gog and Magog, however, locating them north of Cathay. Some scholars have taken this as an oblique and confused reference to the Great Wall of China, which he does not mention otherwise. The Gates of Alexander may represent an attempt by Westerners to explain stories from China of a great king building a great wall. Knowledge of Chinese innovations such as the compass and South Pointing Chariot is known to have been diffused (and confused) across Eurasian trade routes.
The medieval German legend of the Red Jews was partially based on stories of the Gates of Alexander. The legend disappeared before the 17th century.
The Gates of Alexander are most commonly identified with the Caspian Gates of Derbent (Russia) whose thirty north-looking towers used to stretch for forty kilometers between the Caspian Sea and the Caucasus Mountains, effectively blocking the passage across the Caucasus.
Derbent was built around the world's only surviving Sassanid Persian fortress, which served as a strategic location protecting the empire from attacks by the Gokturks. The historical Caspian Gates were not built until probably the reign of Khosrau I in the 6th century, long after Alexander's time, but they came to be credited to him in the passing centuries. The immense wall had a height of up to twenty meters and a thickness of about 10 feet (3 m) when it was in use.
Although the current fortifications date to well after Alexander's death, some scholars postulate that there might have been earlier fortifications built during the Achaemenid Persian Empire (the area has indeed been settled for at least 5,000 years). If this is true, agents of Alexander's empire may have visited or even strengthened them after the Achaemenids were conquered, though Alexander personally never travelled that far north.
See also 
- Kleiber 2007.
- Omrani Rekavandi, H., Sauer, E., Wilkinson, T. & Nokandeh, J. (2008), The enigma of the red snake: revealing one of the world’s greatest frontier walls, Current World Archaeology, No. 27, February/March 2008, pp. 12-22. PDF 5.3 MB. p. 13
- Dathorne, O. R. (1994). Imagining the World: Mythical Belief Versus Reality in Global Encounters. Greenwood. pp. 45–46. ISBN 0897893646. Retrieved February 5, 2013.
- Mikhail Artamonov. "Ancient Derbent" (Древний Дербент). in: Soviet Archaeology, №8, 1946.
- Katarzyna Kleiber, 'Alexander's Caspian Wall – A Turning-Point in Parthian and Sasanian Military Architecture?', Folia Orientalia, vol. 42/43 (2006/07), pp. 173–95