Lonely Planet review
Consecrated in the 13th century and saved from conversion into a mosque by the personal decree of Mehmet the Conqueror, this is the only church in İstanbul to remain in Greek hands ever since Byzantine times. It was named after Princess Maria Paleologina, an illegitimate daughter of Emperor Michael VIII Paleologos.
Maria was sent from Byzantium to marry Hulagu, the Great Khan of the Mongols, in 1265. By the time she arrived in his kingdom he had died (we guess it was a very long trip), so she was forced to marry his son Abagu instead. On Abagu's death she returned to Byzantium and retired to a convent attached to this church.
The church is usually open on weekends. If the doors aren't open, ring the bell on the outside gate to attract the attention of the caretaker.