Hagia Sophia - Istanbul. Not a cathedral, in fact a Mosque. But it used to a church, an early and important one, when the city was called Constaninople and was the seat of the Roman Empire.
Fatih Sultan Mehmet Mosque, Turkey ~ Photo by Ruzgar Photography on 500px
Istanbul, #Turkey. http://reversehomesickness.com/asia/mosques-in-istanbul/