The outside underwhelms; the inside is one of the most astonishing spaces ever built. Justinian's 6th-century church — converted to a mosque in 1453, museum in 1934, mosque again in 2020 — has a dome 56 metres high that appears to float on a ring of light. The surviving Byzantine mosaics, partly revealed through the plaster that once covered them, are extraordinary. Get there when it opens to beat the tour groups.
Hagia Sophia
Justinian's 6th-century masterpiece with a floating dome