You'll feel the weight of fifteen centuries the moment you step inside — the vast dome seems to float above you, defying both gravity and logic. Built as a cathedral in 537 by Emperor Justinian, converted to a mosque after the Conquest, and now functioning as a mosque once more, Ayasofya is Istanbul's single most awe-inspiring building. Stand beneath the dome and look up: golden mosaics shimmer alongside Islamic calligraphy in a space that has witnessed the rise and fall of two empires.