This period gave way to the highly significant Köprülü Era (1656–1703), during which effective control of the Empire was exercised by a sequence of Grand Viziers from the Köprülü family. On 15 September 1656 the octogenarian Köprülü Mehmed Pasha accepted the seals of office having received guarantees from the Valide Turhan Hatice of unprecedented authority and freedom from interference. A fierce conservative disciplinarian, he successfully reasserted the central authority and the empire's military impetus. This continued under his son and successor Köprülü Fazıl Ahmed (Grand Vizier 1661–1676). The Köprülü Vizierate saw renewed military success with authority restored in Transylvania, the conquest of Crete completed in 1669 and expansion into Polish southern Ukraine, with the strongholds of Khotyn and Kamianets-Podilskyi and the territory of Podolia ceding to Ottoman control in 1676.