Ctesiphon

Ctesiphon was an ancient city in modern Iraq, on the eastern bank of the Tigris about 35 kilometres southeast of Baghdad. It served as a royal capital of the Iranian empires for over eight hundred years, in the Parthian and Sasanian periods, and fell in 637 during the Muslim conquest of Persia. It was founded in the late 120s BC on the site of a military camp established across the river from Seleucia by Mithridates I of Parthia, and became the Parthian capital around 58 BC under Orodes II. Gradually it merged with Seleucia and nearby settlements into a cosmopolitan metropolis sometimes called The Cities, in Arabic al-Mada'in, and in the late 6th and early 7th century some accounts listed it as the largest city in the world, covering about 30 square kilometres. Rome captured it several times, including under Trajan in 116, Avidius Cassius in 164 and Septimius Severus in 197. After the Muslim conquest it declined and was depopulated by the end of the 8th century as Baghdad took its place. The most conspicuous remaining structure is the Taq Kasra, or Arch of Ctesiphon.