Among the most important features of Ahmed's reign was his reliance on Köprülüzade Fazıl Mustafa Pasha. Following his accession to the throne, Sultan Ahmed II confirmed Köprülüzade Fazıl Mustafa Pasha in his office as grand vizier. In office from 1689, Fazıl Mustafa Pasha was from the Köprülü family of grand viziers, and like most of his Köprülü predecessors in the same office, was an able administrator and military commander. Like his father Köprülü Mehmed Pasha (grand vizier 1656–61) before him, he ordered the removal and execution of dozens of corrupt state officials of the previous regime and replaced them with men loyal to himself. He overhauled the tax system by adjusting it to the capabilities of the taxpayers affected by the latest wars. He also reformed troop mobilization and increased the pool of conscripts available for the army by drafting tribesmen in the Balkans and Anatolia. In October 1690 he recaptured Belgrade, a key fortress that commanded the confluence of the rivers Danube and Sava; in Ottoman hands since 1521, the fortress had been conquered by the Habsburgs in 1688.