Sultan Hassan Mosque & Madrassa, Cairo

If you're only going visit one mosque in Egypt, then consider this one. Built in 1356 under the orders of Sultan Hassan bin Mohammed bin Qualaun, it was designed to serve not only as a mosque, but also as a religious school for all four Sunni Muslim sects (Shafite, Malakite, Hanefite and Hambelite). The four sects shared the mosque, each teaching their students in one of the four liwans that form a cruciform, but all praying together. While the mosque is beautifully constructed, it had a few hitches. One of the minarets collapsed during construction killing 300 people, another toppled a few centuries later and the dome later collapsed. A nominal admission fee is charged.