Hagia Sophia is a monument to the juxtaposition of East and West that is Istanbul. A former Byzantine Cathedral that was turned into a mosque by Mehmet II with the addition of minarets. This former patriarchal basilica, later a mosque, is now a museum and is famous in particular for its massive dome. 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. One for history junkies.