The Gorgan Wall, a massive fortification stretching 195 kilometers, was constructed by the Sasanian Persians at the end of the fifth century. This impressive structure, made of baked bricks, is known to archaeologists as the "Red Serpent" because of the reddish hue of its bricks. In length, it surpasses the combined total of two Roman walls—those of Hadrian and Antoninus—which together measure about 180 kilometers. The Gorgan Wall is located near the city of Gorgan in the Golestan province, southeast of the Caspian Sea. It was built to block a narrow pass between the sea and the surrounding hills, which was one of the key routes used by nomadic peoples from the Eurasian steppe to enter the heart of Iran. The wall is between 6 and 10 meters wide and includes more than 30 forts along its length, which stretches from the Caspian Sea to the eastern hills. The bricks used in its construction are uniform in size, measuring approximately 40 × 40 × 10 centimeters, with each brick representing about 16 liters of baked earth. A canal, supplied by the Atrak River, ran alongside the wall, serving both as a defensive measure and for irrigation. The forts along the wall were occupied by soldiers, with estimates of the number of troops ranging from 15,000 to 36,000, though no exact records from that time have survived. Archaeologists have determined the wall was built between the middle of the fifth and the beginning of the sixth century using radiocarbon dating and optically stimulated luminescence (OSL), a method that measures the time since mineral grains were last exposed to light. This timeframe points to two potential Sasanian rulers: Yazdgard II (438–457) or Piroz (457–484). The wall is known by several names in different historical traditions, including the barrier of Anushirvân, the barrier of Firuz, or Qizil Alan, which means "Red Serpent" in Turkish. A new city, Dašt-e Qal’e, was established near the wall. The Sasanians likely built the wall to defend against northern enemies, possibly the White Huns. In the Middle Ages, the wall was incorrectly attributed to Alexander the Great and called Sadd-i-Iskandar, although Alexander had died centuries before the wall was constructed. Much of the wall's bricks have been repurposed in local buildings, and several forts are buried under layers of earth, leaving the site open for further archaeological study.