Hagia Sophia, Turkey

Built in 537, the Hagia Sophia first served as a church, later as an imperial mosque and now as a museum with more than 1500 years of history. It had been the largest church in the world for a very long time, until it was replaced by Seville Cathedral in Seville. It is considered to have "changed the history of architecture," for its classic Byzantine style architecture applied to a massive dome. It is one of the landmarks and most famous attractions in Istanbul, attracting millions of visitors every year.