Istanbul is the crown jewel in an important thread of history. When it was known as Constantinople, it was the capital of the Byzantine Empire following the fall of Rome, and it was a significant stop in the Silk Roads. In fact, due to its central location between the East and the West, most classical civilizations played some part in shaping it. Istanbul was also an Ottoman capital until it was dissolved after World War I.
Reminders of each element of its rich history can be witnessed throughout the city, because much of it is marvelously preserved. A great example is the Hagia Sophia, which was a massive church under the Byzantine Empire that was converted into a mosque during the Ottoman rule.