Istanbul is a unique city in the world because it is located on two continents, Europe and Asia, serving as a bridge between them. This geographical location gives it the characteristics of both East and West at the same time. Bosphorus that divides Europe and Asia is one of the world’s busiest waterways connecting the Sea of Marmara and the Black Sea.
Istanbul is the largest city in Turkey, constituting the country's economic, cultural and historical heart. Its commercial and historical center lies in Europe, while a third of its population lives in Asia. Founded on the Sarayburnu heights around 660 BC as Byzantium, it became one of the most significant cities in history. For nearly 16 centuries following its re-establishment as Constantinople in 330 AD, it served as the capital of four empires: the Roman Empire (330–395), the Byzantine Empire (395–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). It was also very important in the advancement of Christianity during Roman and Byzantine times, before the Ottoman Turks, lead by Fatih Sultan Mehmet II, conquered Constantinople in 1453. Renamed Istanbul, it became the third and last capital of the Ottoman Empire. By the mid 1500's, Istanbul, with a population of almost half a million, was a major cultural, political and commercial center. Ottoman rule continued until it was defeated in World War I and Istanbul was occupied by the allies. When the Republic of Turkey was born in 1923 after the War of Independence, Kemal Atatürk moved the capital to Ankara. Nevertheless, palaces and imperial mosques still line Istanbul's hills as visible reminders of the city's previous central role.
Istanbul's strategic position along the historic Silk Road, rail networks to Europe and the Middle East and the only sea route between the Black Sea and the Mediterranean have helped to form its mixed population, although less so since the establishment of the Republic in 1923. Although it was not chosen as the new capital during the interwar period, the city has since regained much of its prominence. The population of the city has increased tenfold since the 1950s, as migrants from across Anatolia have moved to the metropolis and city limits have expanded to accommodate them. Arts festivals were established at the end of the 20th century, while infrastructure improvements have produced a complex transportation network.
Approximately 11.6 million foreign visitors arrived in Istanbul in 2012, two years after it was named a European Capital of Culture, making it the fifth-most-popular tourist destination in the world. The biggest attraction is still its historic center, partially listed as a UNESCO World Heritage Site, but its cultural and entertainment hub can be found on the other side of the Golden Horn, in the Beyoğlu district. Considered a global city, Istanbul is one of the fastest-growing metropolitan economies in the world. It hosts the headquarters of many Turkish companies and media outlets and accounts for more than a quarter of the country's gross domestic product.