Istanbul was the capital of two of the greatest empires of not only their times but also in history: Roman Empire (Byzantine Empire) and Ottoman Empire.
In 324 Istanbul became the capital of the Roman Empire after Emperor Constantin moved the capital from Rome to Byzantium (Istanbul). After the final division of the Roman Empire into West and East Roman Empires in 395 and the ending of the Western Roman Empire in 476, Istanbul became the capital of the Eastern Roman Empire, popularly known as the Byzantine Empire.
In year 361, Istanbul (Constantinople) was the world’s most populous city with a population of 300,000. Throughout most of the Middle Ages, Constantinople was Europe’s largest and wealthiest city.
In 1600, during the Ottoman period, between 400-700,000 people lived in Istanbul making it the world’s second most populous city after Beijing. The population numbers are significant because they relate to power and wealth concentration at those times
Istanbul was also settled by Phoenician, Greek trade colonies from around 700 BC though there are few remains from these periods. Until the conquest of Istanbul by the Turks in 1453, there were also Genoese trade colonies in Istanbul whose major sites were Genoese tower and Yoros castle (in Anadolu Kavagi).
During your visit to Istanbul, you can see Greek, Roman (Byzantine), Genoese, Ottoman historic sights and ancient remains. See also Roman and Ottoman Empire Maps, neolithic discoveries.