Baghdad ( ') is the capital of Iraq and of Baghdad Governorate. With a metropolitan area estimated at a population of 7,000,000, it is the largest city in Iraq. and the Abbasid city was likely built on the site of this earlier settlement.
Baghdad eclipsed Ctesiphon, the capital of the Persian Empire, which was located some 30 km (20 miles) to the southeast, which had been under Muslim control since 637, and which became quickly deserted after the foundation of Baghdad. The site of Babylon, which had been deserted since the 2nd century BC, lies some 90 km (55 miles) to the south.
The city was designed as a circle about 2 km in diameter, leading it to be known as the "Round City". The original design shows a ring of residential and commercial structures along the inside of the city walls, but the final construction added another ring, inside the first. In the center of the city lay the mosque, as well as headquarters for guards. The purpose or use of the remaining space in the center is unknown. The circular design of the city was a direct reflection of the traditional Persian Sasanian urban design. The ancient Sasanian city of Gur/Firouzabad is nearly identical in its general circular design, radiating avenues, and the government buildings and temples at the center of the city.
The roundness points to the fact that it was based on Persian precedents such as Firouzabad in Persia. The two designers who were hired by al-Mansur to plan the city's design were Naubakht, a former Persian Zoroastrian who also determined that the date of the foundation of the city would be astrologically auspicious, and Mashallah, a Jew from Khorasan, Iran."