The Rose or Gül Mosque is located on the Golden Horn, in the Ayakapi neighborhood. Formerly a Byzantine church, the original name and date of the mosque are contested by scholars. Jules Pargoire, writing at the turn of the century, identifies the building as the Church of Euphemia built during the rule of Basil I (867-886) and argues that it was later consecrated to St. Theodosia, a local saint martyred during iconoclast riots. Hartmut Schafer, based on field studies from the 1960s, proposes a later date of construction between 1000 and 1150 and identifies the church as Christos Euergetes.
The origin of the Turkish name is equally uncertain but is often attributed to a Muslim saint named Gül Baba, the alleged owner of the grave placed inside the southern pier. It is not known if the church was in use after 1453, but its crypt was used as a depot by the Ottomans. The conversion to mosque occurred in the last decade of the 15th century. The roof is believed to have collapsed in the 1509 earthquake, calling for partial reconstruction under Selim II (1566-1574). A later restoration dates from the rule of Mahmud II (1808-1839), who added the wooden sultan's lodge.
The church is built atop a vaulted basement, which forms a raised platform for the monument. The walls of the basement are exposed to the southeast and east, where the terrain slopes down towards the Golden Horn. It has a Greek cross or cross-domed plan oriented northwest-southeast. Entering through the wooden porch, built in the 1940s, the nave is preceded by a wide entry hall, capped with a low barrel vault. A triple archway leads into the tall domed nave, flanked by galleries forming the side arms of the Greek cross, and an apsidal sanctuary at its southeast end. The side galleries also have triple archways, but are taller than the entry hall, whereas they share an upper floor with stairs at the southwest end of the entry hall. The level above the entry hall and the right side gallery are used by women and the upper-level of the left side gallery is organized for the Sultan, protected with ornate wooden lattice. The sultan's lodge projects into the nave like a bay window, supported by an adjoining pier and a single wooden console. The side galleries terminate in small chapels flanking the sanctuary on both levels; corridors connect the three apses at the lower level.
The central dome, with its low octagonal drum carried on broad pointed arches, is recognizably Ottoman. The original dome would have rested on a tall drum pierced with windows and the supporting arches would be integrated into the barrel vaults on four sides. The side facades have five tiers of windows, three above the gallery level, that fill the space with light. Some of these windows and those illuminating the sanctuary were opened by Ottoman architects. The orientation of the church allowed for the central placement of the mihrab in the sanctuary. A wooden minber, muezzin's platform and a preacher's pulpit were also added during conversion. The interior is largely plastered, with 18th century Ottoman paintwork.
On the exterior, the tall church has an imposing and symmetrical appearance, animated by the stepped cornice line and the four domes at the corners, forming a visual counterpart to the central dome. The southwest façade is imposing with its three tall apses that are fitted with blind niches of varying heights featuring ornamental brickwork. The church, used as a neighborhood mosque, has a minaret at the western corner that was rebuilt in baroque style after the 1766 earthquake.