As the country's epicenter, Istanbul serves as the gateway to Turkey, sitting fittingly in both Europe and Asia (it's the only city in the world to span two continents!). The historical and cultural influences that have shaped the country are on full display here, homed both literally and metaphorically in the city's iconic monuments of the Old City - just look at the Byzantine Hagia Sophia, Sultan Ahmed Mosque (aka the Blue Mosque), or Topkapi Palace.