Begin your tour of the Sultanahmet district, the heart of old Istanbul, at Hagia Sophia. Built by Constantine the Great in the 4th century and reconstructed by Emperor Justinian in the early 6th century A.D. and designed by Anthemius of Tralles and Isodore of Miletus, the church is one of the marvels of world architecture. Converted into a mosque in 1453, it is now a museum. Its massive dome still dominates the skyline of old Istanbul. It is also famed for its mosaics, including glittering portraits of emperors and empresses and a poignant Virgin and Child. Next, you will visit the Blue Mosque which takes its name from exquisite blue Iznik tiles adorning its interior. Built by Sultan Ahmet I in the early 17th century and designed by a pupil of Sinan, the greatest of Ottoman architects, it is the only imperial mosque with six minarets. Its courtyard is especially grand. The Hippodrome, the stadium of ancient Byzantium, held 100,000 spectators and featured objects from all corners of the empire. Of these, an Egyptian obelisk and a bronze sculpture of three entwined serpents from Delphi survive. Finally, the Grand Bazaar was the commercial heart of the old city and its 4,000 shops are full of treasures including carpets and kilims, silks, jewelry, ceramics, icons and leather goods. Wandering through the Grand Bazaar, indulge in some shopping, Ottoman style.