Hagia Sophia
UNESCO World Heritage Site (Historic Areas Of Istanbul, 1985); Returned To Use As A Mosque In 2020 Under The Directorate Of Religious Affairs · Istanbul, Istanbul
Justinian's church of Holy Wisdom was finished in 537 and held the record for the largest enclosed space on earth for the better part of a thousand years. Its dome, 31 metres across and ringed by forty windows, appears to float because the load is carried outward through half-domes rather than straight down through walls, a trick nobody repeated at this scale for centuries. It was the cathedral of Constantinople, became a mosque in 1453, opened as a museum in 1935 and was returned to use as a mosque in 2020. That last change is the thing to understand before arriving: the ground floor is a carpeted prayer hall with its own rules, foreign visitors are sent up to the gallery on a paid ticket, and everyone is cleared out around the five daily prayers. What survives is a real layering rather than a restoration - gold Byzantine mosaics, vast Ottoman calligraphic roundels and a mihrab set slightly off the building's axis, all in the same room.
Sultan Ahmet Mahallesi, Ayasofya Meydani, Fatih, Istanbul, Istanbul, Turkiye
Open daily as a working mosque. Foreign visitors are routed to the upper gallery on a paid ticket with its own entrance and its own hours, which are shorter than the prayer hall below. Tourist access stops around each of the five daily prayers and for longer at Friday midday. Ramadan and religious holidays change everything. Confirm on the day.