Sasanian defense lines
Both empires gradually absorbed these states, and replaced them by an organized defense system run by the central government and based on a line of fortifications (the limes) and the fortified frontier cities, such as Dara,[6] Nisibis (Nusaybin), Amida, Singara, Hatra, Edessa, Bezabde, Circesium, Rhesaina (Theodosiopolis), Sergiopolis (Resafa), Callinicum (Raqqa), Dura-Europos, Zenobia (Halabiye), Sura, Theodosiopolis (Erzurum),[7] Sisauranon, etc.
According to R. N. Frye, the expansion of the Persian defensive system by Shapur II (r. 309–379) was probably in imitation of Diocletian's construction of the limes of the Syrian and Mesopotamian frontiers of the Roman Empire over the previous decades.
After his successful campaign in Arabia (325) and having secured the coasts around Persian Gulf, Shapur II established a defensive system in southern Mesopotamia to prevent raids via land.
[14] Massive fortification activity was conducted in the Caucasus during the reign of Kavad I (r. 488–496, 498–531) and later his son Khosrow I (r. 531–579), in response to the pressure by people in the north, such as the Alans.
During periods when the Sasanians were distracted by war with the Byzantines or conflicts with the Hephthalites in the east, the northern tribes succeeded in advancing into the Caucasus.
16 m) near Torpakh-Kala has been attributed to Yazdegerd II (r. 438–457) as the first Sasanian attempt to block the Derbent pass, though it may have been a reconstruction of earlier defenses.
[17] With a length of 3,650 m on the north side and 3,500 m on the south and featuring seven gates, massive rectangular and round towers and outworks, the Wall of Derbent connected 30 already existing fortifications.
Later Muslim Arab historians tended to attribute the entire defense line to Khosrow I, and included it among the seven wonders of the world.
For the defense of the Central Asian border, a different strategy was needed: the maximum concentration of forces in large strongholds, with Marv as the outer bulwark, backed by Nishapur.
[4] The defense line was based on a three-tier system that allowed the enemy to penetrate deep into the Sasanian territories and to be channeled into designated kill zones between the tiers of forts.
[2][23] It was on the nomadic route from the northern steppes to the Gorgan Plain and the Persian heartland, probably protecting the empire from the peoples to the north,[24][25] in particular, the Hephthalites.
[29] Recently, Touraj Daryaee has suggested the defensive walls may have had symbolic, ideological and psychological dimension as well, connecting the practice of enclosing the Iranian (ēr) lands against non-Iranian (anēr) barbarians to the cultural elements and ideas present among Iranians since ancient times, such as the idea of walled paradise gardens.
