Ctesiphon, ruined ancient city, 20 mi (32 km) SE of Baghdad, Iraq, on the left bank of the Tigris opposite Seleucia and at the mouth of the Diyala River. After 129 B.C. it was the winter residence of the Parthian kings. Ctesiphon grew rapidly and was of renowned splendor. The Romans captured it in warring against Parthia. It became the capital of the Sassanids in c.224 and a center of Nestorian Christianity. In 637 it was taken and plundered by the Arabs who renamed it, along with Seleucia, al Madain; it was abandoned by them when Baghdad became the capital of the Abbasids. It is now a suburban part of Baghdad. The ruined vault of the great audience hall contains the world's largest single span of brickwork.

Ancient city, central Mesopotamia. Located on the Tigris River, southeast of modern Baghdad, Iraq, it was first a Greek army camp opposite the Hellenistic city of Seleucia. It was the capital of Parthia in the 2nd century BC. Destroyed by the Romans in AD 165, it was resettled by the Persian Sāsānian dynasty in the 3rd century. The Arabs conquered the city in 637 but abandoned it by 763 in favour of a new city, Baghdad. The site is famous for the remains of a gigantic vaulted hall, the Tsubdotāq Kisrā, which has one of the largest single-span brick arches in the world.

Learn more about Ctesiphon with a free trial on Britannica.com.