Rome, Italy

Rome is unparalleled as an architectural timeline, layering 2,700 years of history with unmatched density: the Pantheon (125 AD) stands 500 meters from Bernini's Piazza Navona (1650), which adjoins Mussolini's EUR district (1940s), which neighbors Renzo Piano's Parco della Musica (2002). The city contains 60% of the world's intact ancient Roman structures, including the 1,900-year-old Colosseum, which drew 7.6 million visitors last year.