One of the most important and lasting institutions founded in the Fatimid period was the Mosque of al-Azhar, founded in 970 AD, which competes with the Qarawiyyin in Fes for the title of oldest university in the world. Today, al-Azhar University is the foremost Center of Islamic learning in the world and one of Egypt's largest universities with campuses across the country. The mosque itself retains significant Fatimid elements but has been added to and expanded in subsequent centuries, notably by the Mamluk sultans Qaitbay and al-Ghuri and by Abd al-Rahman Katkhuda in the 18th century.