The ancient Royal road between Susa, the capital of Elam, and Persepolis, the capital of Achaemenid Persia, crosses the large river Pasitigris (modern Karun) at Shushtar. From the Persepolis Fortification Tablets, we know that the old, Elamite name of Shushtar was Šurkutir. Quintus Curtius Rufus, one of the historians of Alexander the Great, describes how the Macedonian king crossed the river on this site in the last month of 331 BCE. Almost six centuries later, when Persia was ruled by the Sasanians, king Shapur I defeated and captured the Roman emperor Valerian, something he celebrated on several reliefs.
Šurkutir: town in ancient Elam, modern Shushtar.
Countless Roman soldiers were taken captive as well. They were ordered to build a bridge at Shushtar and the city of Bishapur. This happened in 260. The Band-e Qaisar ("emperor's bridge") was almost 550 meters long and could also serve as a barrage. The design is Roman. In fact, it is the most easterly bridge constructed by Roman engineers. Perhaps, the bridge was built by soldiers of the Sixth Legion Ferrata, because this unit disappears from our sources after Valerian's defeat.
The monument made it a lot easier to travel from Persia to the west. Six years later, Shapur founded Bishapur ("beautiful city of Shapur") along the road between the Persian heartland and Mesopotamia. The bridge has been repaired several times. During the Sasanian period, the city was an important center of Christianity. It was also a center of some kind of early Industrial Revolution. Many watermills were constructed at Shushtar. Water was led to them through underground tunnels, which were used to capture the city the Arabs, who besieged Shushtar for two years in the 630s.