On the 30th of July 762, Caliph Al-Mansur commissioned the construction of a new city to serve as the capital of his Abbasid Empire. He chose a site on the banks of the Tigris river that had been populated for millennia before the Arab conquest of Mesopotamia in 637 CE. Two designers hired by Al-Mansur planned the layout: Naubakht, a Zoroastrian who determined an astrologically auspicious date under the sign of Leo, and Mashallah, a Jew from Khorasan, Iran. The resulting Round City featured two large semicircles surrounding a circular core with walls up to 44 meters thick and 30 meters high. It took four years to build between 764 and 768, involving over 100,000 workers who laid bricks measuring 18 inches square. Unlike European cities of the time designed as squares or rectangles, Baghdad's circular design reflected ancient Near Eastern urban planning similar to the Sasanian city of Gur. The city included parks, gardens, villas, promenades, fountains, public baths, and thousands of hammams enhancing hygiene. A canal supplied water for drinking and construction while marble steps led down to the river. The Golden Gate Palace stood at the center with a grand 48-meter green dome where only the caliph could approach on horseback. By the middle of the 9th century, Baghdad housed the largest selection of books in the world within its libraries. Estimates suggest the population peaked at more than one million inhabitants during this era. The House of Wisdom became one of the most well-known academies housing scholars like Hunayn ibn Ishaq, al-Khwarizmi, and Al-Kindi. This intellectual hub rivaled Chang'an as one of the largest cities globally until the 930s when it tied with Córdoba.
Destruction And Decline
On the 10th of February 1258, Mongol forces led by Hulegu captured Baghdad after a siege that ruined many quarters through fire, siege, or looting. The Mongols massacred most of the city's inhabitants including Caliph Al-Musta'sim and destroyed large sections of the urban fabric. They also demolished the canals and dykes forming the city's irrigation system which had supported agriculture for centuries. During this period Christians and Shia were tolerated while Sunnis were treated as enemies. The sack of Baghdad put an end to the Abbasid Caliphate and marked what some argue was the final blow from which Islamic civilization never fully recovered. In August 1393, Timur occupied Baghdad having marched there in only eight days from Shiraz. Sultan Ahmad Jalayir fled to Syria where Mamluk Sultan Barquq protected him before killing Timur's envoys. Timur left Khwaja Mas'ud to govern but he was driven out when Ahmad returned. Baghdad was sacked again in 1401 when Timur ordered each soldier to bring back two severed human heads sparing almost no one. The city became a provincial capital controlled successively by the Mongol Jalayirid dynasty from 1400 to 1411, Turkic Kara Koyunlu from 1411 to 1469, Turkic Ak Koyunlu from 1469 to 1508, and Iranian Safavid rulers from 1508 to 1534. Between 1623 and 1638 it returned to Iranian rule before falling back into Ottoman hands. Plague and cholera visitations sometimes wiped out two-thirds of its population during these centuries of instability.