Istanbul is a world center.
Istanbul ( [ ˌɪstænˈbʊl ] , [ - ˈbuːl ] or [ - stɑːn - ] İstanbul [ isˈtɑnbuɫ ] ) , historically known as Constantinople and Byzantium , is the most populous city in Turkey and the country 's economic , cultural , and historic center . The city 's biggest attraction is its historic center , partially listed as a UNESCO World Heritage Site , and its cultural and entertainment hub can be found across the city 's natural harbor , the Golden Horn , in the Beyoğlu district .