Istanbul was Constantinople, now it's Istanbul not Constantinople, and I have to say the city is just about as addictive as the song. I visited Istanbul twice on a trip to Turkey in 2007, and once on a trip in 2014, and it ranks high on my list of favorite cities worldwide. The Ottoman architecture of the Süleymaniye Mosque and the Sultan Ahmed Mosque are extremely impressive, as is the Byzantine architecture of the Hagia Sophia, a former Orthodox basilica that was converted into a mosque and now serves as a museum. The history of the Ottoman empire can be explored through tours of the Topkapı Palace, while below the surface of the Sultanahmet neighborhood are impressive cisterns which I'm glad other reviewers have enjoyed as much as I did. A bus ride to the Edirnekapı neighborhood grants access to the exquisite mosaics of the Chora Church as well as a well-preserved section of the city walls. Perhaps the best way to see the city of Istanbul, though, is by water via a ferry ride across the Bosphorus strait. Istanbul is a city which I'd love to return to some day.

Logistics: Istanbul has an extensive transportation system, including trams, buses, and ferries. Many famous sites in the Sultanahmet neighborhood are easily accessible on foot.