The Great Wall of Gorgan is a Sasanian-era defense system located near modern Gorgan in the Golestān Province of northeastern Iran, at the southeastern corner of the Caspian Sea. The western, Caspian Sea, end of the wall is near the remains of the fort at: ; the eastern end of the wall, near the town of Pishkamar, is near the remains of the fort at: . The title coordinate is for the location of the remains of a fort midway along the wall. The wall is located at a geographic narrowing between the Caspian Sea and the mountains of northeastern Iran. It is one of several Caspian Gates at the eastern part of a region known in antiquity as Hyrcania, on the nomadic route from the northern steppes to the Iranian heartland. The wall is believed to have protected the Sassanian Empire to the south from the peoples to the north, probably the White Huns. However, in his book Empires and Walls, Chaichian (2014) questions the validity of this interpretation using historical evidence of potential political-military threats in the region as well as the economic geography of Gorgan Wall's environs. It is described as "amongst the most ambitious and sophisticated frontier walls" ever built in the world, and the most important of the Sassanian defense fortifications. It is long and wide, and features over 30 fortresses spaced at intervals of between . It is surpassed only by the walls systems of Great Wall of China as the longest single-segment building and the longest defensive wall in existence. Name Among archaeologists the wall is also known as "The Red Snake" (Turkmen: Qizil Alan) because of the colour of its bricks. In Persian, it was popularized by the name "Alexander Barrier" ( Sadd-i-Iskandar) or "Alexander's Wall", as Alexander the Great is thought by early Muslims to have passed through the Caspian Gates on his hasty march to Hyrcania and the east. It is also known as the "Anushirvân Barrier" ( Sadd-i Anushiravan) and "Firuz/Piruz Barrier" (), and is officially referred to as "Gorgan Defence Wall" (). It is known as or to local Iranian Turkmens. Description The barrier consists of a wall, long and wide, with over 30 fortresses at intervals of between . The building materials consist of mud-brick, fired brick, gypsum, and mortar. Clay was also used during the early Parthian period. Mud-bricks were more popular in the early period