Istanbul, and formerly known as Byzantium and Constantinople, is the most densely populated city in Turkey and the economic, historic and cultural center of the country. Istanbul is situated between the Sea of Marmara and the Black Sea. It is one of the world's largest cities by population and the largest city in Europe, with its around fifteen million residents. While there, you can visit Istanbul's biggest attraction — its historic center, which is listed as a UNESCO World Heritage Site. There you can enjoy arts, music, film and cultural festivals, along with many cultural and historical sites.