Ancient city founded by the Parthians, located on the Tigris River southeast of modern Baghdad, Iraq. It served as the winter capital of the Parthian empire and later of the Sasanian empire. The site is famous for the remains of a gigantic vaulted hall, the Taq Kisra, which is traditionally regarded as the palace of the Sassanian king Khosrow I (reigned 531-579 AD) and Shapur I (reigned 241-272 AD). The hall has one of the largest single-span mud-brick arches in the world.