The architectural feature which gives to this building a peculiar interest, in the study of the development of planning and construction, is the more complete fusion of the basilican type of plan with a domical system of roofing which it presents than is found in any other example of a similar combination. On the west, where the ground retains its original level, stands the old atrium, though much modified by Turkish repairs and alterations. It had covered arcades on the north, south, and west sides, but only the outer walls of the northern and southern arcades, with some portions of their inner walls, and three complete vaulted bays at the northern end of the western arcade, are Byzantine. The walls, vaults, and piers in other parts of the arcades are Turkish. There is no trace of the west door which, under ordinary circumstances, would form the main entrance to the atrium, but a Byzantine doorway, now built up, is found close to the narthex, in the outer wall of the south arcade. The area of the atrium has been, moreover, greatly reduced by the erection, on its four sides, of an inner range of Turkish vaulting. Five doors led from the atrium to the narthex, but only the central and the northernmost of these doors are now open, the latter entrance still retaining its original architrave and cornice of white marble, with the usual mouldings and a cross worked on the crowning member of the cornice. The present entrance to the church, however, is on the north side of the building, through a porch that leads down a sloping Turkish passage to the western end of the north aisle. The narthex is in five bays, the two terminal bays having cross-groined vaults, the three central, vaults of a domical character with blunt rounded groins at the springing. The whole vaulting surface of the narthex was once covered with mosaics exhibiting mainly a geometrical pattern. From the narthex three tall arched openings conducted to the nave, and one opening to each aisle. But the direct communication between the narthex and the northern aisle is now cut off by the insertion of the Turkish entrance to the church, although the old doorway to the aisle remains complete. The nave is divided into two large bays of equal breadth but unequal length, the western bay being the shorter. In the latter the arches which support its roof are, to the east and west, semicircular, while those to north and south are roughly elliptical, springing from the same level and rising to the same height as the semicircular arches, but being of shorter span. These elliptical arches extend to the outer walls of the church, thus partaking of the character of short barrel vaults.