Salah ad-Din's reign marked the beginning of the Ayyubid dynasty, which ruled over Egypt and Syria and carried forward the fight against the Crusaders. He also embarked on the construction of an ambitious new fortified Citadel (the current Citadel of Cairo) further south, outside the walled city, which would house Egypt's rulers and state administration for many centuries thereafter. This ended Cairo's status as an exclusive palace-city and started a process by which the city became an economic center inhabited by common Egyptians and open to foreign travelers. Over the subsequent centuries, Cairo developed into a full-scale urban center. The decline of Fustat over the same period paved the way for its ascendance. The Ayyubid sultans and their Mamluk successors, who were Sunni Muslims eager to erase the influence of the Shi'a Fatimids, progressively demolished and replaced the great Fatimid palaces with their own buildings. The Al-Azhar Mosque was converted to a Sunni institution, and today it is the foremost center for the study of the Qur'an and Islamic law in the Sunni Islamic world.