Istanbul, the seaport and one of the most beautiful Turkish cities, the largest in the country and the fourth largest in terms of the number of inhabitants in the world, is not the capital of Turkey . Once it was or rather, was the capital of the Byzantine Empire and was called Constantinople . Then Byzantium fell, the city was renamed to Istanbul, and made the main city of the then new Ottoman Empire . But in the 1920s it ceased to exist . @ In October 1923, Mustafa Kemal Atatürk proclaimed the establishment of Turkish and also about its new capital Ankara . So now all the capital functions are performed by this city .