inhabitants: 14.4 mio
Istanbul is the largest city in Turkey and the only transcontinental stop on this trip (part of Istanbul is in Europe, part is in Asia).
Founded around 660 BC as Byzantium, it was called Constaninople for nearly sixteen hundred years and served as an imperial capital for the Roman and Byzantine, the Latin and Ottoman empires before becoming known as Istanbul. The Bosphorus, which connects the Sea of Marmara to the Black Sea, divides the city into a European an Asian side. Istanbul is is a big exciting metropolis and full of stories.
To start, you can take a walk down Istanbul's famous Istiklal Street to Galata, the other side of the Golden Horn and home of the Genoese colony from the 6th to 15th century.
One of the most famous sights is the Hagia Sophia, originally a Greek Orthodox basilica (church), later an imperial mosque, and now a museum, with its 107 columns and Empereror's Door (a door which used to be for the Byzantine emperors only)
Directly opposite is Sultanahmet Mosque (The Blue Mosque) with its six minaret towers, various domes and balconies and carpet-covered floors.
Another highlight is a visit to one of the covered markets, like the Grand Bazaar with over 3,000 stalls selling carpets, leather goods, jewelry, spices, Turkish delight, tea, and much more.
Ferries connect the European to the Asian side. Or you could take a trip across the Sea of Marmara to the Princes' Island Büyükada.