The only city that straddles two continents, and for fifteen centuries the capital of empires. Byzantium, Constantinople, Istanbul, three names for one of the great cities of the world.
The historic peninsula holds the heart of it: Hagia Sophia and the Blue Mosque facing each other across a garden, Topkapi Palace above the water, and the covered lanes of the Grand Bazaar and Spice Bazaar. Most of it sits within walking distance, layered Roman, Byzantine and Ottoman on the same streets.
But the city lives on the Bosphorus. Ferries cross between Europe and Asia all day, fish sandwiches grill by the Galata Bridge, and the waterfront neighbourhoods from Karakoy to Kadikoy are where the real life of the city happens. Give it at least three days, and cross to the Asian side at least once.