The residences near Kufa proved unsuitable so, on August 1, 762, the second Abbasid caliph, al-Mansur, decided to move the capital to Medina al-Salam (Baghdad). The site, near Ctesiphon, was chosen for its easy riparian communication with Mesopotamia, the Persian Gulf, and northern Syria, as well as its important land routes to the Iranian plateau, southern Syria, and the Hejaz. Work on the new capital was completed four years later in 766/767. As with the first Abbasid capitals near Kufa, nothing remains of Abbasid Baghdad, which is entirely covered by the modern city. Extensive descriptions in medieval texts. however, have allowed scholars in modern times to reconstruct the city's general plan. About 2.7 kilometers (l .7 miles), the Round City was surrounded by a double set of sturdy, mud-brick walls, and a broad moat fed by the Tigris river. The walls were pierced, at the inter-cardinal points, by four gates - the Khorasan Gate on the northeast, the Basra Gate on the southeast, the Kufa Gate on the Southwest, and the Damascus Gate on the northwest - from which roads led to the four quarters of the empire.

Each of the four gates to al-Mansur's Round City possessed a complex, bent :nuance passage designed to guard it against violent attack. Each gate was surmounted by an elevated chamber reached by staircases or ramps. Each of the chambers was crowned by a dome, and the whole 25 meter structure was topped by a weathervane in the shape of a human figure. The caliph used these rooms as audience halls when he wished to view anyone who might be approaching or whatever lay beyond the city walls. The audience halls also marked the extension of his personal domain and authority over the extremities or tile city.

Four major avenues lined with shopping arcades and other buildings led from the gates into the interior of the city. Adjacent to the wall on the interior walls, all outer ring of residences for the caliph's family, staff, and servants. An Inner ring of residences housed the arsenal, tile treasury, and government offices. The innermost zone of the city was a broad esplanade in which stood the police station, the Friday mosque, and the caliph's palace.

Round City of Baghdad

 
In 762, the Abbasid caliph al-Mansur began construction, but scholars have reconstructed the round plan. Two sets of mud-brick walls and four axial gates protected a ring of residences and government offices. In the center of the ring stood the caliph's palace and the adjacent mosque.

The mosque was a square hypostyle structure measuring 200 cubit, (approximately 100 meters, 330 feet) on each side, with an open interior courtyard. Adjacent to the mosque was the palace; located in the exact center of the city. It covered covered times tile area of the mosque. At the back of the palace, a reception hall. (Iwan) measuring 15x10 meters (50x33 feet) led to a domed audience chamber on each side. Above it was another donned audience hall, known to contemporaries as the °Qubbat al-Khadra," often translated as the "Green Dome°' but more accurately rendered as tile Heaven," thereby making reference to an ancient tradition of associating the ruler with the heavens. The top of this dome stood 40 meters above the ground and was itself crowned by a weathervane in the shape of a horseman.

Contemporaries considered the horseman the crown of Baghdad, a symbol of the region, and a monument to the Abbasids. The revolving horseman was also a convenient metaphor for the caliph's power and authority. It was said that the sultan saw the figure with its lance pointing toward a given direction, he knew that rebels would appear, before word had reached him. Like a weathervane, the horseman was supposed to predict storms before they blew in. The collapse of the Qubbat al-Khadra and its horseman during a storm in 941 was indeed an omen: within four years the Buyids entered Baghdad and established themselves as "protectors' of the Abbasid caliphs.

The Round City was built to Separate the caliph from his Subjects. Several settlements stood Outside the walls: a great army camp stood at Harbiya, markets were located in al-Karkh, and al-Mansur's son al-Mahdi built a subsidiary camp for his troops on the cast bank of the Tigris at Rusafa. The Round City soon failed to achieve its original purpose, as the population settled thickly around it, and even the administrative core was quickly transformed into a normal urban entity. This was particularly apparent following the siege of-8 12/813 during the civil war between Harun al-Rashid's sons, when tile original Khorasani army was replaced by new units. The victorious Caliph al-Mamun moved his palace from the Round City to a suburban estate on the east bank of' the Tigris, and the Round City was swallowed up by the new metropolis developing on the west bank Sections of the original city wall remained visible for centuries, but no trace of the Round City has been found in modern times.
 

Palace of Ukhaidir

Ukhaidir is the best-preserved palace from the early Abbasid period. The exterior is protected by a large wall built of limestone rubble in heavy mortar, with round towers at the corners, semicircular towers along the sides, and quarter-round towers protecting the axial gates. In the center, a courtyard opens onto a Iwan, with a Square hall behind it. One each side is a self contained residence arranged around a smaller court, and to the east is a a bath complex.

Baghdad city circular form and centralized planning, with the caliph's palace in the exact center of the city and the mosque adjacent to it, invite speculation about the city's intended cosmic significance as the center of a universal empire. It has been speculated, for example, that al-Mansur modeled his city on such earlier round-shaped royal foundations as Firuzabad, in Fars. As attractive as this hypothesis and others may be, there is no contemporary evidence to either support or disprove them. In any event, within a few decades, if not years, of its foundation, the administrative center had been transformed from a large-scale palace into a rich and vibrant industrial and commercial center.

Air view of Firuzabad, Fars, Iran, c. 224-241
The Sassanian emperor, Ardashir I, founded the city of Gur (modern Firuzabad) in southwestern Iran. Round cities of this type may have inspired the Abbasid caliph al-Mansur, in the 8th century, to build his capital Baghdad as a round city.