Fatih Mosque was a Byzantine church in Bithynian city of Trigleia. Picturesquely set at the center of an historic town on the south shore of the Sea of Marmara, it has long been recognized as a significant early example of the cross-in-square church type. The Byzantine name of the monument, which appears to have been converted into a mosque in the sixteenth or seventeenth century, is uncertain. The most natural identification would be with the monastery of Trigleia whose abbot, St. Stephen the Confessor, suffered persecution in the reign of Leo V. It can now be securely placed in the early ninth century by dendrochronology, with a post quem date of 799 for the wood analysed from the building.
During the occupation of Trigleia by the Greek army in 1920-22, it was converted into a church, during which time mosaics were uncovered but not photographed. Today, traces of mosaic may be seen in the soffit of the south tribelon that consists of large cubes, alternately black and white, forming a checkerboard pattern. The four columns of the atrium, capped by reused sixth-century capitals, appear to be standing in their original position.
Recently several elements of its original design have clarified. The naos is close to square in overall plan, with a dome just under 5 in. in diameter, raised on a tall drum above four columns. The crossarms are covered by barrel vaults. The corner compartments are somewhat uneven, isolated by projecting pilasters and covered by ovoid domical vaults. The pastophoria were quite large - the diakonikon is now missing - with their lateral walls projecting beyond the width of the naos. The bema has an extra bay before the apse, which was curved on the interior and polygonal on the exterior, opened by three windows. The pastophoria each included a setback before the apse, which was semicircular on both interior and exterior. To the west is a broad, barrel-vaulted narthex, preceded by a colonnaded portico.
Exposed remains of architectural sculpture and additional marbles littering the site suggest that the original building was lavishly fitted out. Much of the sculpture, including the capitals of the naos and closure panels, is reused from the sixth century, although some, including the capitals of the lateral arcades and some of the cornice patterns, may be of the ninth century. The interior was originally decorated with mosaics, the presence of which was noted during the period of Greek occupation in 1920-22, when the building was briefly reconverted to a church. Mosaics in a simple grid of oversized tesserae survive in the soffits of the south arcade and east windows. A restoration of 1995-96 opened the arcades on the north and south sides of the naos. Fragments of opus sectile were uncovered at the same time.
Byzantium in the Iconoclast Era (ca 680–850): The Sources by Leslie Brubaker, John Haldon
Some Churches and Monasteries on the Southern Shore of the Sea of Marmara by Cyril Mango and Ihor Ševčenko