A Hellenistic city, Seleucia-on-the-Tigris, was founded on the west bank of the ancient Tigris river by Greeks in the 3rd century BC. When the Parthians rose to power in ancient Iran, they built their own city, Ctesiphon, on the opposite side of the river.
This became the capital city of the Parthians in the 1st century BC and was later the capital of the Sassanians in the 3rd century AD. Under the Sassanians it experienced a great expansion in size and population due to the irrigation of the east bank through the construction of the Nahrawan canal - perhaps the greatest Sassanian irrigation project.
The city was ethnically diverse and had architecturally distinct districts. Canals linked together the districts.
The old city with its round walls - a typical feature of Parthian and Sassanian cities - contained the Sassanian 'white palace', but much of this part of the city is now lost due to the Tigris river having changed its course. The ancient suburbs have survived, however, and the 'Taq-i Kisra' - 'the Hall of Khusrau' - still stands. It is a great vaulted iwan (hall) built either by King Shapur I (AD 241-272) or by one of the two kings called Khusrau (AD 531-578 and 590-628).
