Ctesiphon, the ancient city famous for the Taq Kasra arch, is in Iraq.
Ctesiphon stood on the eastern bank of the Tigris River, about 32 kilometers southeast of modern Baghdad. It developed into a major royal city under the Parthian Empire and later became one of the principal capitals of the Sasanian Empire, which ruled Persia and neighboring regions before the Arab-Muslim conquests.
Its best-known surviving monument is Taq Kasra, also called the Archway of Ctesiphon. The enormous vaulted iwan is widely regarded as the largest single-span brick vault built before the modern era. Its original palace complex was much larger than the surviving arch suggests.
Ctesiphon is often confused with nearby Seleucia, a Hellenistic city on the opposite side of the Tigris. The two urban centers eventually formed a connected metropolitan area, but they were founded in different periods and are treated as separate archaeological sites.