wall and replaced all the marble tiles in the prayer hall. Sultan al-Nasir Muhammad also undertook major restoration work for the mosque in 1328. He demolished and completely rebuilt the unstable qibla wall and moved the Bab al-Ziyadah gate to the east. Much of that work was damaged during a fire that burned the mosque in 1339. Islamic art expert, Finbarr Barry Flood, describes the Bahri Mamluks' attitude towards the mosque as an "obsessive interest" and their efforts at maintaining, repairing, and restoring the mosque were unparalleled in any other period of Muslim rule. The Arab astronomer Ibn al-Shatir worked as the chief muwaqqit ('religious timekeeper') and the chief muezzin at the Umayyad Mosque from 1332 until he died in 1376. He erected a large sundial on the mosque's northern minaret in 1371, now lost. A replica was installed in its place in the modern period. The Minaret of Jesus was burnt down in a fire in 1392. The Mongol conqueror Timur besieged Damascus in 1400. He ordered the burning of the city on 17 March 1401, and the fire ravaged the Umayyad Mosque. The eastern minaret was reduced to rubble, and the central dome collapsed. A southwestern minaret was added to the mosque in 1488 during the reign of Mamluk sultan Qaitbay. Ottoman era Damascus was conquered from the Mamluks by the Ottoman Empire under Sultan Selim I in 1516. The first Friday prayer performed in Selim's name in the Umayyad Mosque was attended by the sultan himself. The Ottomans used an endowment system (waqf) for religious sites as a means to link the local population with the central authority. The waqf of the Umayyad Mosque was the largest in the city, employing 596 people. Supervisory and clerical positions were reserved for Ottoman officials while religious offices were held mostly by members of the local ulema (Muslim scholars). Although the awqaf (plural of "waqf") were taxed, the waqf of the Umayyad Mosque was exempted from taxation. In 1518, the Ottoman governor of Damascus and supervisor of the mosque's waqf, Janbirdi al-Ghazali, had the mosque repaired and redecorated as part of his architectural reconstruction program for the city. The prominent Sufi scholar Abd al-Ghani al-Nabulsi taught regularly at the Umayyad Mosque starting in 1661. The khatib (preacher) of the Umayyad Mosque was one of the three most influential religious officials in Ottoman Damascus, the other two being the Hanafi mufti