Hagia Irene - the Church of Holy Peace - was one of the few churches not converted to a mosque after the fall of Constantinople in 1453. It was used as an arsenal instead.
Inside the First Courtyard of the Topkapi Palace stands the Church of St. Irene, originally built in the 4th century, burned down in 532 and rebuilt in 548, now open as a museum and, because of its extraordinary acoustics, as a concert hall.