Hagia Sophia museum, Istanbul
Hagia Sophia, finished in 537 A.D., is considered the epitome of Byzantine architecture. It was the largest cathedral ever built in the world for nearly a thousand years, until the completion of the Seville Cathedral in 1520.
For the ~500 years after the conquest of Istanbul by Ottomans, it served as the principal mosque of Istanbul. It's architecture served as the model for many mosques in Istanbul, including the famous Blue Mosque. (summarized from Wikipedia)
Today it serves as a museum, complete with the Christian and Muslim elements uncovered.
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