The final sight on Sultanhamet’s holy trinity tour is the Basilica Cistern or “Sunken Palace,” a subterranean well located southwest of the Hagia Sophia. The cistern, first constructed by Constantine then rebuilt and enlarged by Justinian, was used to store water for the Great Palace of Constantinople and the Topkapi Palace after the Ottoman conquest of 1453. The macabre underground site, reminiscent of a cathedral was neglected for centuries but has been recently renovated for public tour. Open daily 9–5.

Across the park from the Hagia Sofia is the early-17th-century Sultanahmet, or Blue Mosque. Famous for its scandalous six minarets (only the Prophet’s mosque at Mecca was allowed that many) and blue Iznik tiles, which inspired its nickname, the building is particularly dramatic at dusk.

In 1853, the reigning sultan abandoned the Topkapi Palace for the Dolmabahce Palace. The glittery, over-the-top interiors were designed by French decorator Séchan, who also masterminded the interiors of Paris’s Opera Garnier. Closed Monday and Thursday.

Built between 532–537, during the reign of Emperor Justinian I, when the city was Constantinople, the Hagia Sofia had the largest dome in the world for a thousand years until it was eclipsed, in 1436, by Florence’s Duomo. Once one of the most important Christian churches, safeguarding religious treasures such as fragments of the True Cross and various relics of saints, it was ransacked by the Crusaders in 1204 (and converted to an Islamic mosque more than two centuries later, in 1453). Inside you can view fascinating Christian mosaics. Closed Monday.

Designed by Mimar Sina is the small Rüstem Pasha Mosque, whose Iznik tiled walls are even more amazing than those of the Blue Mosque.

This is the largest, and some say the most beautiful, mosque in the city. Built in 1557 for Sultan Süleyman the Magnificient, it’s considered a masterpiece of the architect Mimar Sina.

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You’ll need at least half a day to properly explore the Topkapi Palace. The administrative center of the Ottoman Empire for nearly four hundred years, from 1465 to 1853, the palace is a lavish complex of beautifully preserved tiled buildings and picturesque courtyards. Don’t miss the Harem (which requires a separate ticket and a fairly long wait), the eye-popping gemstones and Ottoman riches showcased in the Imperial Treasury and the excellent Archaeological Museum. Closed Tuesday.

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Underneath Istanbul are seven ancient reservoirs and numerous natural springs that supplied the citizens with water. One of the most famous and easily most dramatic is the Yerebatan Sarnici, or Basilica Cistern. A narrow stone staircase leads to an eerily lit space the size of an enormous cathedral filled with water, giant ghostly fish and a forest of 336 columns. If it looks familiar that’s because James Bond rowed across it in From Russia with Love.