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Photo: Diego Delso, CC BY-SA 4.0 via Wikimedia Commons
Vast underground cistern built by Justinian in 532 to store the city's water, the largest and best-known of the hundreds constructed. It's a cathedral-like space with 336 richly decorated columns. Time was you explored in a little rowing boat, but tourist numbers have soared and water levels have dropped, so now you follow a boardwalk, with lights, piped music and art installations. It's sometimes a film location, and stages concerts. But the crowds rob it of atmosphere and you might prefer to seek out one of the less-visited cisterns.
The Basilica Cistern, or Cisterna Basilica, is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul, Turkey. The cistern, located 150 metres (490 ft) southwest of the Hagia Sophia on the historical peninsula of Sarayburnu, was built in the 6th century during the reign of Byzantine emperor Justinian I. Today, it is kept with little water for public access inside the space.
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Bathhouse built in 1556 by Mimar Sinan, the chief Ottoman architect, at the request of Hürrem Sultan (Roxelana), wife of Suleiman the Magnificent. It was built over the former public baths of Zeuxippus, abandoned around 713. Restored in 2011, it's now an upmarket spa.
Aya Sofya
The third and grandest of the churches on this site, completed in 537 AD. The 30-m diameter dome covers what was for over 1000 years the largest enclosed space in the world, only surpassed in 1520 by Seville cathedral. It used "pendentive" architecture to throw the dome's weight onto four stout pillars, a design that inspired many Eastern Orthodox churches and Ottoman mosques. It became a mosque after the Ottoman conquest in 1453, remaining so until 1931. It was then a museum until 2020 when it was again proclaimed a mosque. That means you must work around prayer times and dress appropriately. Very cheap disposable headscarves are available for purchase outside the building.
Blue Mosque
The defining image of Istanbul, dominating the skyline with its great dome and six minarets. Completed in 1617 for Sultan Ahmed I, it's still a working mosque, so dress appropriately and avoid prayer times. Enter via the courtyard on the SW side. You step (shoeless) into the blue-tiled prayer hall beneath the main dome and its semi-domes. The mihrab is of finely carved marble, well-lit, and the minbar (pulpit) next to it is visible from almost all parts.
Built in 1741, the last major hamam opened in the Ottoman era. It has ornate architecture, great service and a stellar cast of famous visitors. Prices to match, and only tourists now use it.
İstanbul Arkeoloji Müzeleri
Turkey was slow to protect its antiquities and to display them properly, but in 1867 Sultan Abdülaziz toured the museums of Paris, London and Vienna and saw what was needed. The main collection is in a grand neo-classical building of 1891, with two annexes for ancient art and Islamic art. Exhibits include Sumerian tablets, pieces of the wall of Babylon, Roman statues, and the Alexander Sarcophagus. There are very detailed historical explanations in both Turkish and English. There is also quite an accurate exhibition about the historical layers and epochs of the city of Troy. If you are passionate about history, you can spend many hours here, so bring a supply of water and food (the museum café is quite expensive). In the summer, on Saturdays and Sundays, they have concerts in the courtyard.
Şerefiye Sarnıcı
This was rediscovered in 2010 when an overlying building was demolished. It was built circa 430 AD to store water brought in by Valens Aqueduct. It's smaller than the better-known Basilica Cistern, but better lit, more atmospheric and less touristy.