Ctesiphon was the capital city of the Parthian and Sasanian Empires (247 BCE??24 CE and 224??51 CE respectively). It was one of the great cities of late ancient Mesopotamia. Its most conspicuous structure remaining today is the great archway of Ctesiphon.

It was situated on the eastern bank of the Tigris across from where the Greek city of Seleucia stood and northeast of ancient Babylon. Today, the remains of the city lie in Baghdad Governorate, Iraq, approximately 35 km (22 mi) south of the city of Baghdad.

Ctesiphon was the largest city in the world from 570 CE, until its fall in 637 CE, during the Muslim conquests.

The arched iwan hall at Taq Qasra, open on the facade side, was about 37 m high, 26 m across and 50 m long, the largest man-made, free standing vault constructed until modern times.