(3rd to 7th century AD). In 2005 a team excavated samples of charcoal from the many brick kilns along the wall, and samples from the Gorgan Wall and the smaller Wall of Tammishe (location of a drowned fort at the northern end: ; location of a fortlet or watchtower at the inland end: ); OSL and radiocarbon dating indicated a date for both walls in the late 5th or 6th century AD. These dates suggest that the current wall, at least, is Sassanid rather than Parthian, and that the current structure did not yet exist, some 800 years earlier, in the time of Alexander the Great (died 323 BC). If Alexander encountered a barrier at this location it was a predecessor of the current wall. If we assumed that the forts were occupied as densely as those on Hadrian's Wall, then the garrison on the Gorgan Wall would have been in the order of 30,000 men. Models, taking into account the size and room number of the barrack blocks in the Gorgan Wall forts and likely occupation density, produce figures between 15,000 and 36,000 soldiers. Even the lowest estimate suggests a strong and powerful army, all the more remarkable as our investigations focused just on 200km of vulnerable frontier, a small fraction of the thousands of kilometres of borders of one of the ancient world's largest empires. Derbent Caspian Gate A similar Sasanian defence wall and fortification lies on the opposite, western, side of the Caspian Sea at the port of Derbent, in the Republic of Dagestan, Russia. There the remains of a line of fortifications run inland for some from the shore of the Caspian Sea () to what is today an extraordinarily well preserved Sassanian fort () on the first foothills of the Caucasus mountains. Derbent and its Caspian Gates are at the western part of the historical region of Hyrcania. While the fortification and walls on the east side of the Caspian Sea remained unknown to the Graeco-Roman historians, the western half of the impressive "northern fortifications" in the Caucasus were well known to Classical authors. See also Hadrian's Wall Gates of Alexander Iranian Architecture Notes FURTHER READING Chaichian, Mohammad Empires and Walls: Globalization, Migration, and Colonial Control, chapter 3, 2014 Brill: Leiden . External links Aerial view in the Mazandarani wiki Gorgan's Great Red Snake (Science Daily, February 2008) Secrets of the Great Wall of Gorgan