The ancient city of Ctesiphon was at one time the capital of the vast Persian empire and in the 6-7th centuries it was the largest city on the world, a 600k people melting pot of religions and cultures. After the Abbasid Muslim invasion in the 8th century, the city was abandoned and the capital of the new empire was moved to Baghdad to the north. The only structure surviving today is the Taq Kasra aka Arch of Ctesiphon. It is to this day the largest single span brick arch in the world! While it is officially closed for restoration, there is always a way in past the check-point and restriction-laden layers of Iraqi bureaucracy.