Istanbul is the largest city in Turkey, forming the country's economic, cultural, and historical heart. With a population of 13.5 million, the city is at the center of the second-largest metropolitan area in Europe and thesecond largest city proper in the world. Istanbul's vast area of 5,343 square kilometers (2,063 sq mi) is coterminous with Istanbul Province, of which the city is the administrative capital. Istanbul is a transcontinental city, straddling the Bosphorus—one of the world's busiest waterways—in northwestern Turkey, between the Sea of Marmara and the Black Sea. Its commercial and historical center is situated in Europe, while a third of its population lives in Asia.
Founded on the Sarayburnu around 660 BC as Byzantium, the city now known as Istanbul developed to become one of the most significant cities in history. For nearly sixteen centuries following its reestablishment 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 instrumental in the advancement of Christianity during Roman and Byzantine times, before the Ottomans conquered the city in 1453 and transformed it into an Islamic stronghold and the seat of the last caliphate.Although the Republic of Turkey established its capital elsewhere, inAnkara, remnants of Istanbul's previous central role still remain highly visible across the city, with palaces and imperial mosques lining its hills.
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 foster an eclectic populace, although less so since the establishment of the Republic. Overlooked for the new capital during the interwar period, the city has since regained much of its prominence. The population of the city has increased