If you have time to see just one building in Istanbul, head to the Hagia Sophia. For a thousand years, it was the largest building on earth, although the Hagia Sophia’s power didn’t derive from gross size alone. The Byzantine historian Procopius witnessed this miracle rising from the ground: ‘A spherical-shaped domed standing upon this circle is exceedingly beautiful; from the lightness of the building, it does not appear to rest on a solid foundation, but to cover the place beneath as though it were suspended from heaven by the fabled golden chain’. Built by the great Byzantine emperor Justinian (it was finished in AD 537), it’s hard to overestimate its importance over the subsequent centuries in the imaginations of both friends and foes of Constantinople, generating the kind of awe in contemporaries that New York’s skyline would for those who have never seen a city before. But the real magic, as Procopious understood, was inside. The idea of the interior was to be a place of awe that in its spaciousness, the improbability of its dome, the gleam of its gold mosaics in the candlelight, and the scent of incense, transported, almost literally, the worshipper to a heavenly place. Such responses belong to another time, but the architecture and the decoration – particularly the Empress Zoe and Comnenus mosaics – are still otherworldly.