Hagia Sophia
For nearly a thousand years the largest cathedral in the world, Hagia Sophia (Ayasofya) has stood at the heart of Istanbul since 537 AD. Commissioned by the Byzantine emperor Justinian, later converted into an imperial Ottoman mosque, and today once again a working mosque and monument, its vast 55-metre dome, shimmering gold mosaics and towering marble columns remain one of humanity's greatest architectural achievements. Your guide unpacks its Christian and Islamic layers side by side — the single most important landmark in the city.