Istanbul, Turkey

Istanbul is the only major city in the world spanning two continents, with roughly 15.5 million residents living across Europe and Asia along the Bosphorus strait. Founded as Byzantium in 660 BC, the city served as the capital of three empires — Roman from 330, Byzantine, and Ottoman from 1453. Hagia Sophia, completed in 537, holds a 31-metre dome that remained the largest in the world for nine centuries. Topkapı Palace housed Ottoman sultans for 380 years beginning in 1465, while the 1616 Blue Mosque displays 20,000 İznik tiles. The 1471 Grand Bazaar still draws visitors through 61 covered streets lined with 4,000 shops.