The so-called “Church of St. Theodosia”, now known as Gül Camii (Turkish ‘Rose Mosque’), was a Byzantine church in the quarter of Dexiokratous of Constantinople near the Golden Horn Walls. The identity and dating of the church are difficult to determine, as it was significantly altered during both the Byzantine and Ottoman eras. While it has traditionally been identified as the Monastery of St Theodosia, this has often been rejected, with other dedications including the Monastery of Christ Euergetes being suggested.

The church’s traditional identification is partly due to its close proximity to Aya Kapı (Turkish ‘Holy Gate’), which was probably originally known as the Gate of St. Theodosia. Many other traditions and legends surround the history of the church-mosque. According to one tradition, the church was filled with roses to commemorate St. Theodosia, on May 29, 1453 when the city was captured by the Ottomans. It has also been associated with a Muslim saint known as Gül Baba (“Father Rose”) whose tomb is supposedly inside the church. By the Late Ottoman era, there was even a tradition associating the tomb of Constantine XI Palaiologos. Shortly after the Ottoman conquest of Constantinople in 1453, the substructure of the church was used as a depot by the Ottoman navy. It is unclear when it was converted into a mosque, with records suggesting it was used as a mosque by the late 15th century or even later in the 16th century. It was extensively renovated by Murat IV (1622-1640) after being damaged by an earthquake. It was again restored during the reign of Mahmut II (1808-39).

The church’s architecture is complicated since it has been significantly altered over the centuries.  Even distinguishing between Byzantine and Ottoman architectural features is a complex issue. The church has traditionally been dated to the 11th or 12th century, due in part to the recessed-brick masonry of sections of the church and its substructure. The church rests on a high terrace of a massive vaulted subterranean basement. The church is cross-in-square plan with a triple apse. It currently has a windowless Ottoman dome resting on four massive irregular piers, though it would originally have had windows in the dome and would have been supported by columns. It seems that a gallery was added during the Palaiologan era with additional Ottoman alternations being made later dates. It is now situated over the narthex and extends over the side aisles of the naos. There are small chambers on the upper level between the passageways between the main apse and the side apses. While the chamber to the northeast is now inaccessible, the chamber to the southeast is accessible through a spiral staircase.