Ctesiphon (tĕs´Ĭfŏn´, tē´sĬ–), 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 BC 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.