Hagia Sophia, Istanbul, Turkey (UNESCO)
Originally the Hagia Sophia was built as an orthodox church, later changed to a mosque and now is a museum in Istanbul.
Construction started in 532 and until 1453, with a brief intermezzo as Roman Catholic Cathedral, it was the seat of the Patriarchate of Constantinople (Istanbul's former name). Until 1931 it was a mosque and then turned into a museum by the first Turkish president Kemal Atatürk.
This beautiful card was sent by my parents during their Mediterranean cruise in October 2013