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Photo: fusion-of-horizons, CC BY 2.0 via Wikimedia Commons
Built as the Church of Saints Sergius and Bacchus around 530 AD, the same time as Hagia Sophia, but their styles differ. Transformed into a mosque in the early 1500s, it's beautifully decorated, with fine marble details, and has a pleasanter atmosphere than the larger busy mosques. The adjoining madrasa houses craft shops.
The Little Hagia Sophia mosque, formerly the Church of Saints Sergius and Bacchus, is a former Greek Orthodox church dedicated to the Syrian Christian Saints Sergius and Bacchus in Constantinople. Its date of construction is disputed, with some scholars favouring a mid-520s date with groundbreaking by 527 and others preferring to date its construction between 532 and 536, and converted into a mosque during the Ottoman Empire.
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Sokollu Mehmet Pasha was the grand vizier and de facto ruler of the Ottoman Empire during its 16th century golden age. He commissioned this mosque, one of three in the city to bear his name, but the sloping site was a challenge. He hired the best, Mimar Sinan, who completed it in 979 AH / 1571 AD, with the slope resolved by a fronting courtyard above shops and a medrese. The interior has outstanding İznik blue tiles, and three fragments of the Black Stone (Hajar al-Aswad) of the Kaaba: above the main entrance, minbar and mihrab.
Established in 1777, this is one of the cheaper options. It's marble throughout and the fittings could be original. It has a mixed main section and a separate female section for the scrubbing.
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.
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.
Yerebatan Sarnici
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.
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.