Istanbul, formerly known as Constantinople, is a mega city that sits in both Europe and Asia, divided by the Bosporus. Although it is the largest city of Turkey, it is not the capital. It is the largest city in Europe by population. It is also the 3rd largest European city in size. It was the capital city of the old Ottoman Empire until 1923. The city has been known since ancient times by the older names Byzantium and Constantinople. Being a seaport, Istanbul is the main trade center of Turkey.