Sassanid Borders and Frontier Defences

The Sasanian Empire (224-651 CE) faced the Roman and later Byzantine empires across northern Mesopotamia, while its northeastern approaches led toward the Caspian Sea and Central Asia. Sasanian defenses combined fortified cities that controlled crossings and roads, mountain corridors that narrowed travel, long barriers that joined forts and waterworks, and inland bases where field forces assembled. The eastern and southern edges also depended on negotiations with neighboring rulers and Arab clients. Each system had to connect defensible ground with water, food, transport and the people who kept a garrison supplied.

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Cities and crossings on the Roman frontier

In Mesopotamia, now divided among Iraq, Syria and southeastern Turkey, the fortified cities of Nisibis and Dara faced one another across a contested frontier. Nisibis passed from Roman to Sasanian control in 363 under a treaty made after Emperor Julian’s death. Under the Sasanians, it became an advanced base opposite Roman and Byzantine defenses, and a target for cross-border raids. Merchants continued to use it as a crossing into Byzantine territory, while the authorities imposed safeguards against espionage. A frontier city thus served military and commercial movement at the same time.

Dara, near modern Mardin in southeastern Turkey, belonged to the Eastern Roman Empire. Emperor Anastasius founded it as a fortified garrison town in 507, and Justinian later strengthened its defenses. It stood close to Sasanian territory and became a focal point in wars with the Sasanian king Khosrow I. After a long siege in 540, the Sasanian army withdrew when its departure was purchased; in 573 a Byzantine force surrendered after a four-month siege and Dara fell to Khosrow. Its changing control made it a strategic anchor on routes through upper Mesopotamia.

Closing the Caucasian corridor

At the Caucasus, the Caspian shore and mountains squeezed north-south travel through a narrow strip. Derbent, in Dagestan in present-day Russia, occupied a coastal plain only about three kilometres wide between the western Caspian Sea and the Caucasus. The major Sasanian barrier dates to the fifth century; UNESCO separately identifies substantial sixth-century construction at the city walls and citadel. Two parallel walls ran from the shore to the mountain citadel, with the town between them. Another wall continued west over the mountains, while the eastern walls extended into the sea to protect the harbor. Gates concentrated passage through the corridor linking the Caspian shore, the town, the citadel and the mountain route.

Building and supplying the Gorgan Wall

Farther east, the Great Wall of Gorgan crossed northern Iran’s Gorgan Plain for almost 200 kilometres. The archaeological record places its major construction in the fifth century, with forts occupied into the seventh. Researchers also identify an earlier chain of forts along a riverbank, probably replaced by the linear wall after a relatively short interval. The wall’s fired bricks rested on an earthen bank beside a ditch. Its builders dug canals along the route to provide water for making bricks; supplier canals crossed the Gorgan River via qanats, underground channels, and fed water to the construction route. The channels followed the plain’s natural gradient, linking water management to the construction of a military barrier.

UNESCO records 38 forts along the wall. Excavations at Forts 2 and 4 found sixth-century ovens, storage pits and provisions in large jars. Fort 2 had long barracks widened by annexes. These remains show how a wall’s routine depended on food stored nearby and places where soldiers could sleep and cook. A reservoir formed where the wall crossed the Sari Su River may also have deepened the water at that vulnerable crossing, making an approach harder and protecting a bridge from erosion. The excavators present this as a likely function among several possible uses for the reservoir.

Bases behind the frontier

Large bases stood behind the immediate frontier as well. Iraj Castle, or Qal’eh Iraj, lies near the ancient city of Rayy, close to modern Tehran. Excavators date its construction probably to the late fourth or early fifth century. Its roughly 175-hectare enclosure had massive walls, hundreds of rooms built into the perimeter and a defensive ditch, while the interior contained few permanent buildings. The research team interprets the open space as a place where a field army could assemble in tents before moving toward northern fronts. This layout differs from Gorgan’s wall forts, whose barracks, ovens and stores served troops stationed along the barrier.

Eastern and southern approaches

Khorasan, a broad eastern military region encompassing parts of modern Iran, Afghanistan, Turkmenistan and neighboring lands, confronted shifting powers and local rulers. Defeat by the Central Asian Hephthalites in the fifth century forced Sasanian rulers to pay tribute; Peroz died in battle against them in 484. A different arrangement operated in the south. The Lakhmid rulers based at Hira, near Kufa in present-day Iraq, served as Sasanian clients and helped police Arab groups to the north. The Lakhmids helped extend Sasanian influence across the northern Arabian approaches.

Campaigns could also follow maritime and desert routes. The ninth-century historian al-Tabari reports that the Sasanian king Shapur II crossed the Persian Gulf in 325, moved through historical Bahrain, a region on the northeastern Arabian mainland, and Hajar near the modern Hofuf oasis, and destroyed wells in Yamama, in east-central Saudi Arabia, before advancing toward Medina.