Istanbul, Turkey, was founded about 660 BC as Byzantium. The city has changed names numerous times and was initially known as Byzantium. It was the capital city of the Eastern Roman, Byzantine, and Ottoman empires and was led by rulers such as Alexander the Great, Septimius Severus, and Constantine the Great. The city became known as Istanbul after falling to Ottoman rule in the 15th century. It once carried the title of the world's most crowded city. Today, it has a population of more than 15-million. Interesting fact: part of the city lies in Europe, and the other part is in Asia..