Kairaouine Mosque, Fez
Concierge.com's insider take:
Built in 857 A.D. and spanning approximately 10,760 square feet, the immense Kairaouine was Morocco's largest mosque until the 1990swhen the Hassan II Mosque was built in Casablanca. Non-Muslims are not admitted inside the building, but through the open door, visitors can view dozens of Moorish arches and a brilliantly painted ceiling. At the start of the second millennium, this mosque housed the Western World's first university, and its legendary scholars include Pope Sylvester II, the mathematician responsible for introducing Arabic math to Europe.