Hagia Sophia ( Ayasofya-i Kebir Cami-i Şerifi )
The Church of the Divine Wisdom (Hagia Sophia in Greek) in Sultanahmet, is one of the most impressive and important buildings ever constructed.
Its wide, flat dome was a daring engineering feat in the 6th century, and architects still marvel at the building's many innovations.
A Bit of History
Ayasofya was the greatest church in Christendom, and was meant to be. According to Prof. Robert Osterhout, it was built to surpass the gigantic Church of St Polyeuchtoserected by Julia Anitzia, scion of the line of Theodosian emperors.
Julia meant her church, a "recreation" of the Temple of Jerusalem, to symbolize her wealth, power and legitimate claim to the throne of Byzantium. Justinian had to out-build her to establish his own legitimacy—and he did.
His church remained the largest church ever built until St Peter's Basilica was constructed in Rome 1000 years later. (Julia's church, by the way, was destroyed by an earthquake. You can see a few pitiful ruins of it near the traffic under/overpass between the Istanbul Belediye Sarayı [City Hall] and Aqueduct of Valens [Bozdoğan Kemeri]
Being the world's most impressive building, it's no wonder that Mehmet the Conqueror proclaimed it a mosque soon after his conquest of the city from the Byzantines in 1453.
It served as Istanbul's most revered mosque until 1935 when Atatürk, recognizing its world-historical significance, had it proclaimed a museum, but in 2020 was reverted back to a mosque.