On the eastern banks of the Nile River Caliph Omar founded his enclosed city of Al-Fustat in 640 after the death of the Prophet Mohamed sent the Muslim armies conquering neighboring lands. The plain and severe mosque of the Prophet in Medina (Mosque of 'Amr ibn al-'As) was built. The later Abbasids preferred their suburb of Al-Aksar to the north, while partial independence with Governor Ahmed Ibn-Tulun (870) meant the development of Al-Qatai in the northeast, including the magnificent Mosque of Ibn-Tulun, although Abbasids destroyed much of the area when they returned. This was also a time when the Jewish population founded their Ben Ezra Synagogue (in 882). The Fatimids created the quarters of Al-Qahira and Al-Azhar from 969, and although their name and many towers, walls and gates survive, their two grand palaces do not. 1172 saw the arrival of Saladin of the Ayubbids, and the first Mameluke mosques.
While most Egyptians converted to Islam in the 12th Century, Cairo and Egypt have a 12-million-strong population of Coptic Christians (despite centuries of persecution from rulers and Crusaders), the oldest Christian denomination founded by Saint Mark the Apostle, and their base is Old Cairo.
In addition to this, there is the presence of the Greek Orthodox Church with the Church and Monastery of St. George, though the most famous church in Old Cairo is the Hanging Church, St. Virgin Mary's or 'the Staircase Church' (due to its 29 approaching steps) founded in 690, which is built over part of the walls of a Babylon Fortress gate.
Source: http://en.wikipedia.org/wiki/Old_Cairo
http://whc.unesco.org/en/list/89
http://wikitravel.org/en/Cairo/Old_Cairo