The road heads westwards, entering the third and most extensive of the walled cities of Merv, Sultan Kala. The Abbasid commander Abu Muslim promoted urban development around the Majan Canal, a kilometer to the west of the walls of Gyaur Kala, possibly because of growing difficulties over access to water in the existing city, as the ground level rose with each new phase of building. The old city of Gyaur Kala gradually declined in importance, and the archaeological evidence suggests that it became an industrial suburb of Sultan Kala. The new city reached its peak under the Seljuks. It was walled in the 11th century under Malik Shah. During the reign of Sultan Sanjar, when Merv was one of the most important cities of the world, further northern and southern suburbs were walled, giving the city a total area of some 600ha. The city had declined by the time of the arrival of a large Mongol force in 1221, but its walls still posed a formidable obstacle for the invaders. The Mongols did not, however, need to attempt to breach them: the defenders of the city negotiated a surrender under which they would be spared and opened the gates to the Mongol army. The latter promptly forgot about its deal and massacred everyone. Not much is yet known about the nature of the occupation of Merv in the Mongol period, though there is evidence that the city remained in use and even minted its coins. To the east side of the road, after passing into Sultan Kala, is a walled citadel known as the Shahriar Ark, built in the 12th century to enclose a palace and administrative complex, and the residences of the urban elite. The ruins of the palace are close to the center of this site: the building was based around a central courtyard, surrounded on each side by iwans. The best-preserved building in the Shahriar Ark, however, stands to the northeast of the palace. It is a rectangular building some 20m long, and surviving to around 8m in height. Its external walls have a distinctive design of vertical corrugations. The interior is distinctive too, with many square niches. The building is known as a keeper khana, or ‘pigeon house’, based on one theory as to the use to which the niches were put, but there is no academic consensus as to the purpose of this unusual building.