What is the significance of Istanbul as described in the text?

Istanbul is described as a major city located at the crossroads of Europe and Asia, on both sides of the Bosphorus Strait. It was formerly known as Constantinople during the Byzantine Empire and Tsargrad in Rus'. The text highlights its historical significance with mentions of the Hagia Sophia, underground water reservoirs, aqueducts, the Blue Mosque, and Topkapi and Dolmabahce palaces.