Derbent
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.
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