The capital city of the Roman Empire between 330-395 AD, in Byzantine times, Byzantium was considered the world centre of civilsation for 600 years. Then having been known under the name of Constantinople, after the declaration of the Republic in 1920s was renamed Istanbul. All business in Constantinople has historically been linked to the Bosphorus, dividing the European side of the city from the Asian side of the city. Istanbul is now the third largest city in the world with a population in excess of 11 million and the only metropolis in the world situated on two continents.