The historic peninsula where Istanbul began and where the concentration of world-class monuments would be implausible if it were not true. Hagia Sophia, the Blue Mosque, Topkapi Palace, the Basilica Cistern, and the Grand Bazaar are all within a fifteen-minute walk of each other, layered atop Roman, Byzantine, and Ottoman ruins. The area is heavily touristed, which means the restaurants are often mediocre and overpriced — eat elsewhere and return for the monuments.
Morning visits beat the crowds; the mosques and Hagia Sophia are most atmospheric in early light when tour groups are still at breakfast.