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 to Syria. To the north-east the Khorasan Gate lay close to the Tigris, leading to the bridge of boats across it. The four straight roads that ran towards the center of the city from the outer gates were lined with vaulted arcades containing merchants' shops and bazaars. Smaller streets ran off these four main arteries, giving access to a series of squares and houses; the limited space between the main wall and the inner wall was due to Mansur's desire to maintain the heart of the city as a royal preserve. By 766, Mansur's Round City was complete. The ninth-century essayist, polymath, and polemicist al-Jahiz said. "I have seen the great cities, including those noted for their durable construction. I have seen such cities in the districts of Syria, in Byzantine territory, and in other provinces, but I have never seen a city of greater height, more perfect circularity, more endowed with superior merits or possessing more spacious gates or more perfect defenses than Al Zawra (Baghdad), that is to say, the city of Abu Jafar al-Mansur." The city had an impressive array of basic services and employed a large staff of civil servants. These included night watchmen, lamplighters, health inspectors, market inspectors (who examined the weights and measures as well as the quality of goods), and debt collectors. It also had a police force with a police chief who lived in the caliph's palace. Destruction and abandonment The Round City was partially ruined during the siege of 812–813, when caliph Al-Amin was killed by his brother, who then became the new caliph. It never recovered; its walls were destroyed by 912, nothing of them remain, there is no agreement as to where it was located. Centre of learning (8th to 9th centuries) Founder, caliph al-Mansur of the Abbasid caliphate, chose the city's location because of its critical link in trade routes, mild climate, topography (critical for fortification), and proximity to water. All of these factors made the city a center for culture and knowledge. Baghdad is set on the Khurasan Road, which was an established meeting place for caravan routes from all directions. During the construction of the