The Nilometer, built in 247 Hegira (861 A.D) during the reign o the Abbasid Caliph Mutawakil-ala-Allah, is considered an important Islamic monument in Egypt. It contains significant architectural elements and calligraphic inscriptions. According to the historian Ibn-Khallikan, it was Ahmad Ibn Mohammed al-Hasib who built it, and it was completed in the Arab month of Rajab, 247 Hegira, as mentioned earlier. The name of Caliph Mutawakil was inscribed on one of the friezes inside the Nilometer, but it was removed by the architects of Ahmad Ibn Tulun when the Nilometer was renovated in 259 Hegira. It was restored in 485 Hegira, (1092 A.D) by Badr al-Jammali, minister of the Fatimid Caliph Mustansir. The mosque, known as Al-Miqyas (the mosque of the Nilometer), was built next to it. Al-Zahir Baybars built a dome over the well of the Nilometer, which was restored later by Sultan Al-Ashraf Qaytbay. During the Ottoman period many other renovations were carried out. More recently the Committee for the Preservation of Arab Monuments restored it 1925 following the collapse of the Nilometer's base. It was then that the pyramidal cover, presently found on top, was added.