|The red pin is Kufa, the purple pin|
is Baghdad, about 170 km north.
Although the date of its founding is accepted as 30 July 762, building Baghdad took 100,000 workers from 764 to 768. The location of the city on the Tigris was beneficial: the abundance of water throughout the city encouraged growth that allowed for all residents to have easily accessible water. By the 9th century, Baghdad had grown to be the largest city in the Middle East, with a population between 300,000 and 500,000.
The entire city complex was originally built as a circle about 19 kilometers in diameter and was nicknamed al-Mudawara ["Round city"]. The wall was built to last: it was about 44 meters thick at the base, and about 30 meters high. Al-Mansur brought together artists as well as engineers to build his capital, and the circle included parks and promenades, gardens, and a mosque at the center. The city walls had four gates called Basra, Khorasan, Kufa and Syria, named for the places that the highways from those gates led to.
Baghdad became a center of knowledge. The first university in the world is considered to be the Bayt al-Hikma ["House of Wisdom"], founded during the reign of Caliph Harun al-Rashid (786 - 809). Scholars here made it a goal to translate all Greek works available to them, which made much of Classical Era learning available to the West through Arabic translations.
The strength of the Abbasid Caliphate started to deteriorate due to religious and regional strife. A grandson of Ghengis Khan (but not Kublai, well-known to Marco Polo fans) managed to sack Baghdad in 1258, destroying much of what made it great in learning and art and religion. The city was further devastated in 1401 by Tamerlane.