Istanbul, historically known as Constantinople, is the largest city in Turkey and 5th largest city proper in the world with a population of 12.8 million, also making it the second largest metropolitan area in Europe by population, and the largest metropolitan city proper. Istanbul is also a megacity, as well as the cultural, economic, and financial center of Turkey. The city covers 39 districts of the Istanbul province. It is located on the Bosphorus Strait and encompasses the natural harbour known as the Golden Horn, in the northwest of the country. It extends both on the European (Thrace) and on the Asian (Anatolia) sides of the Bosphorus, and is thereby the only metropolis in the world that is situated on two continents. Istanbul is a designated alpha world city.
During its long history, the city previously served as the capital city of the classical (330–395), "Byzantine" (395–1204 & 1261–1453), and "Latin" (1204–1261) Roman Empires and the Ottoman Empire (1453–1922), prior to the selection of Ankara as the capital of the new Republic of Turkey during the Turkish Civil War. The city was chosen as joint European Capital of Culture for 2010. Historic areas of Istanbul were added to the UNESCO World Heritage List in 1985.
Information based on http://en.wikipedia.org/wiki/Istanbul