The earliest buildings to be used as churches in the ancient world were chiefly of two types: the basilica and the centralized building. The basilica, developed in Hellenistic and Roman times for a variety of public purposes, was for several centuries the plan most widely adopted for ordinary churches. It is a long rectangular building divided by two rows of columns into three parts, a wide central nave flanked by an aisle on each side; at the east end of the nave is a semicircular projection or apse. The entrance, at the west and opposite the apse, is generally preceded by a vestibule or narthex, which in turn opens into a large arcaded courtyard or atrium. Dozens of early examples of this plan are to be found in all parts of the Roman Empire, but in Istanbul only one pure basilica has survived, the church of St. John of Studius, built in 449–50, now partly ruined, but whose basilical structure is still clearly visible.
The early basilicas had pitched roofs and flat ceilings. Later, chiefly in the reign of Justinian, an innovation was made by introducing a dome. Two ancient examples in Istanbul survive intact: Haghia Eirene and Haghia Sophia. The nave of the former is covered at the east by a large dome, at the west by a smaller, slightly elliptical domical vault; otherwise it is a very typical basilica. In Haghia Sophia the enormous central dome is supported to east and west by two semidomes of equal diameter and there are other modifications which to a superficial view conceal its essentially basilical plan.
The other type of classical building sometimes used for churches was of a centralized plan, round or polygonal. In Istanbul there remain the very scanty ruins of a few such buildings of a very early period, but the most famous and beautiful is somewhat later in date, the church of SS. Sergius and Bacchus built by Justinian in 527. In form it is an octagon inscribed in a rectangle with a projecting apse and a large central dome. Between the eight piers which support the dome, there are pairs of columns on the ground floor and gallery level, thus making continuous ambulatories except in front of the apse. SS. Sergius and Bacchus closely resembles the contemporary church of St. Vitale at Ravenna.
The 250 years after Justinian (roughly 600 to 850) were a period of decline and confusion, unfavourable to the arts. When architecture began to revive in the ninth century, a new type of church building came into vogue, generally known as the cross-domed church. In this type a central dome is surrounded in the axes of the building by four long barrel vaults resting on four strong corner piers, thus forming an internal cross; on three sides are aisles and galleries, so that the exterior is rectangular. At the east end the wide central apse is flanked by two smaller side apses; thenceforward three apses became habitual, demanded by the developed ritual; and at the west there is the usual narthex. In Istanbul the clearest and grandest example of this type is the church of St. Theodosia (Gül Camii), probably dating from the eleventh century.
Another type, often considered as a mere development of the plan of the cross-domed church, though it may have had an independent origin, has been called by several names, but we shall identify it as the four-column church because its most striking internal feature is the four columns which here take the place of the corner piers of the earlier type as supports for the dome. These churches are all small and tall, more or less square on the exterior, but preserving the cruciform plan within. There are no galleries, except sometimes over the narthex, but the four corners of the cross are occupied by bays domed or with domical vaults on high drums; these, together with the central dome, form a quincunx, by which name this type is sometimes known. The four-column church first appears in Istanbul in the ninth or tenth century and thereafter became almost standard; its small size was suitable to the declining revenues of the shrinking Empire, while its interior form provided ample areas for mosaic and fresco decoration. In Istanbul no less than eight examples survive, of which perhaps the most typical are the two churches which form parts of the complex building of St. Saviour Pantocrator (Zeyrek Camii).
All the Byzantine churches in Istanbul are built of brick, including Haghia Sophia, and they were generally little adorned on the exterior, depending for their effect on the warm brick colours of the walls and the darker areas of windows which were usually plentiful and large. Towards the end of the Empire in the thirteenth and fourteenth centuries, exteriors were sometimes enlivened by polychrome decoration in brick and stone, seen at its best and most elaborate in the façade of the outer narthex of St. Theodore (Kilise Camii) or in that of the palace of Constantine Porphyrogenitus (Tekfur Saray). As if to compensate for the relative austerity of the outside, the interior of the churches blazed with colour and life. The lower parts of the walls up to the springing of the vaults were sheathed in marble, while the vaults, domes and upper walls were covered in gold mosaic. The most magnificent example of marble revetment is that in Haghia Sophia where a dozen different kinds of rare and costly marbles are used, the thin slabs being sawn in two and opened out so as to form intricate designs. The Great Church was of course unique, though there may have been a few others of Justinian’s time almost equally lavishly covered with marble. But even the humbler and smaller churches of a later period had their revetment, largely of the common but attractive greyish-white Proconnesian marble quarried from the nearby Marmara Island. Most of the churches have now lost this decoration, but an excellent example survives almost intact at St. Saviour in Chora (Kariye Camii).
The mosaics of the earlier period seem to have consisted chiefly of a gold ground round the edges of which, emphasizing the architectural forms, were wide bands of floral decoration in naturalistic design and colours; at appropriate places there would be a simple cross in outline. Quantities of this simple but effective decoration survive from Justinian’s time in the dome and the aisle vaults of Haghia Sophia. It appears that in Haghia Sophia at least there were originally no pictorial mosaics. In the century following Justinian’s death, however, picture mosaics became the vogue and an elaborate iconography was worked out which regulated what parts of the Holy Story should be represented and where the pictures should be placed in the church building. Then came the iconoclastic age (711 to 843) when all these pictural mosaics were ruthlessly destroyed, so that none survive in Istanbul before the mid-ninth century. From then onward there was a revival of the pictorial art, still in the highly stylized and formal tradition of the earlier period, and all the great churches were again filled with holy pictures. A good idea of the stylistic types in vogue from the ninth to the twelfth centuries can be seen in the examples that have been uncovered in Haghia Sophia.
But in Istanbul the most extensive and splendid mosaics date from the last great flowering of Byzantine culture before the fall. At the beginning of the fourteenth century were executed the long cycles of the life of the Blessed Virgin and of Christ in St. Saviour in Chora, which have been so brilliantly restored in recent years to their pristine splendour by the Byzantine Institute. To this date also belong the glorious frescoes in the side chapel of that church and another series of mosaics, less extensive but hardly less impressive in the side chapel of St. Mary Pammakaristos (Fethiye Camii). The art of these pictures shows a decisive break away from the hieratic formalism of the earlier tradition and breathes the very spirit of the Renaissance as it was beginning to appear at the same date in Italy. In Byzantium it had all too short a life.