The Caliph al-Mansur founded the epicenter of the empire, Baghdad, in 762 CE, as a means of disassociating his dynasty from that of the preceding Umayyads (centered at Damascus) and the rebellious cities of Kufa and Basrah.

Thousands of architects, engineers, legal experts, surveyors, carpenters, blacksmiths, diggers, and ordinary laborers from across the Abbasid empire were brought in to survey, measure and excavate the foundations. Ya'qubi, in his Book of Countries,reckoned there were 100,000 workers involved. "They say that no other round city is known in all the regions of the world," according to Al-Khatib al-Baghdadi. Four equidistant gates pierced the outer walls where straight roads led to the centre of the city. The Kufa Gate to the south-west and the Basra Gate to the south-east both opened on to the Sarat canal – a key part of the network of waterways that drained the waters of the Euphrates into the Tigris. The Sham (Syrian) Gate to the north-west led to the main road on to Anbar, and across the desert wastes to Syria. To the north-east the Khorasan Gate lay close to the Tigris, leading to the bridge of boats across it.