In 754, al-Mansur became the Abbasid Caliph, succeeding his brother as-Saffah in rule. By 762 he had commissioned the construction of a new eastern capital, choosing Baghdad as his site. Builders from the greater region came to work for him by the thousands. What followed would be a grand model of urban design notable for its circular plan. The city was designed with ash drawings onto the ground for al-Mansur to view prior to construction, which began that same year. By its completion in 766-7, it has been posited that the Round City measured 2000 meters in diameter. It featured four main gates, equidistant from each other: the southwest gate was the Kufa Gate; the southeast was Basra; the Khurasan Gate extended to the northeast and the Damascus Gate to the northwest. The walls were constructed out of mud brick with reed supports, while the domes and vaults were composed in baked brick.
An outer wall and main wall fortified the city. Each gateway included a vaulted chamber with a domed second storey and was built into the main enclosure, which was supported by approximately 28 towers between each one. They also highlighted a bent outer entrance opening up into a courtyard across which would be the main entrance.
A residential area with houses and stores partitioned into quadrants was located between the central headquarters and the fortifications. Four barrel-vaulted arcades ran the distance of the city from the outer entrances to the gateways of the central palatial and governing section. An outer and inner ring road flanked these quarters with eight to twelve roads running through each quadrant. The central governing area contained the palace, a mosque, residences for the princes, government offices, and a kitchen. A Green Dome featuring the figure of a rider holding a spear was situated at the highest point of the palace in the exact center of the city.
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Ettinghausen, Richard and Grabar, Oleg. The Art and Architecture of Islam 650-1250, 75-79. New Haven and London: Yale University Press 1987.
Lassner, J. "The Caliph's Personal Domain The City Plan of Baghdad Re-Examined." In The Islamic City, edited by Hourani and Stern, 103-118. Oxford: Bruno Cassirer Ltd., 1970.