Istanbul (, or ; ), historically known as Constantinople and Byzantium, is the most populous city in Turkey and the country's economic, cultural, and historic center. Istanbul is a transcontinental city in Eurasia, straddling the Bosphorus strait (which separates Europe and Asia) between the Sea of Marmara and the Black Sea. Its commercial and historical center lies on the European side and about a third of its population lives on the Asian side. The city is the administrative center of the Istanbul Metropolitan Municipality ( with Istanbul Province), both hosting a population of around 14.7 million residents. Istanbul is one of the world's most populous cities and ranks as the world's 7th-largest city proper and the largest European city. 

Founded under the name of "Byzantion" (Βυζάντιον) on the Sarayburnu promontory around 660 BCE, the city developed to become one of the most significant in history. After its reestablishment as "Constantinople" in 330 CE, it served as an imperial capital for almost 16 centuries, during the Roman and Byzantine (330–1204 and 1261–1453), the Latin (1204–1261), and the Ottoman (1453–1922) empires. 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 Ottoman Caliphate.
how many centuries has it been an imperial capital?
 imperial capital for almost 16 centuries