Church of St. Mary of the Mongols, Istanbul

The Church of Saint Mary of the Mongols is an Eastern Orthodox Church that is located in the Fatih district in the neighborhood of Fener. It stands on the summit of a slope, and overlooks the Golden Horn. The Church of Saint Mary of the Mongols is the only surviving Byzantine church of Constantinople, which the Ottomans never converted into a Mosque, and it has always been open to the Greek Orthodox Christians. Its conversion into a mosque was saved by a personal decree of Mehmet the Conqueror.

The church is not open to the public, but the structure has tremendous historical significance, being the only original surviving Byzantine church in Istanbul. At the time that it was built, in the 7th century, the church was dedicated to Saint Eustolia and has been significantly renovated ever since.

The original mosaics and paintings that were made during the 11th -13th centuries have been removed from the inside of the church, but it is still richly decorated. The church complex consists of a tetra-conch plan, comprising a central dome which has been enclosed by a tower. The dome rests on 4 semi-domes. This kind of design is called a unicum, and is typically found in the Byzantine architecture of Constantinople.