Church of Constantine in Rome. S. Costanza: Interior

While the nave resembled that of-St John Lateran, the west end was markedly different: it was not so much a congregational meeting-place as the precinct of a martvr-shrine. The transept, taking the form of a small nave set at right angles to the main nave, was something of a novelty. It came to be a convenient and familiar element in Gothic architecture but in the fourth century it was provided as an exceptional feature to meet the special purpose of accommodating a throng of worshippers. It could, however, be claimed as a Hellenistic form derived from the cult room set in front of pagan hero-shrines. As regards the form of the sanctuary at St Peter’s, Gregory of Tours is again informative:

The tomb is placed under the altar and is kept entirely on its own. But, if anyone desires to pray, the screens which surround the place are unbolted and he has access to the point just above the tomb. A small window may be opened there, so he puts his head inside and asks for whatever his need requires.

The site of the altar is not exactly known; it may indeed have been movable until Gregory the Great arranged for Mass to be celebrated 'over St Peter's body' In that case, the altar would be set somewhere in front of the shrine at the time of services while the clergy, emerging from the apse, ranged themselves on either side with the people behind them.

Constantine's churches at Rome, combining classic simplicity of design with richness of internal decoration, set the pattern for later builders. But all, whether large basilican meeting halls or compact, centralized martyr-shrines, survive only as fragments beneath later reconstructions. They are, however, sufficiently preserved to indicate the ways in which Constantine faced the problem of connecting a large hall, adapted for congregational worship, to the relics of the martyr whose presence was a focus for devotion. The general pattern of S. Lorenzo and S. Sebastiano was followed in the church of St Marcellinus and St Peter, adjoining the tomb of Helena, mother of Constantine, on the Via Labicana, and, again, in the case of St Agnes on the Via Nomentana.

Nothing is known concerning the details of St Agnes' martyrdom: it may be supposed that she was a victim of the final persecution in the time of Diocletian. At any rate she found her resting-place in a tiny shrine contained within a catacomb taken over by Christians from its previous use as a pagan burial- place. A church was constructed over the top layer of this catacomb, apparently a fair-sized hall with galleries projecting above ground. But the site was transformed when Constantine, or perhaps his son Constantine II, set up a large basilica, with the customary aisles and clerestory, alongside. Pope Honorius I, about 630 ad, again modified the whole arrangement by building a new church in place of, rather than incorporating, Constantine's basilica.

The precise location of St Agnes’ shrine was obscured, but presumably she lies directly beneath the massive altar which, as though by substitution, took on the appearance of a sarcophagus and bears, carved on its front face, the figure of a female saint with hands outstretched in prayer. It is possible to compare the great basilica of SS. Nereus, Achilles and Petronilla, constructed at the end of the fourth century and partially sunk into the earth in order to include a range of tombs immediately below the paving of the floor.

Another type of Constantinian building was the mausoleum, a name applied by the Romans to exceptionally elaborate tombs constructed for imperial personages. The sepulchre still partially preserved in the Castel Sant' Angelo was built about the year 135 ad for the emperor Hadrian and served as the resting-place of the emperors and their relatives for seventy or eighty years. A circular tower some 68 metres in diameter sprang from a vast square base, the whole structure being elaborately decorated with colonnades, statues and a glistening surface of Parian marble.

At the top rose a pyramid, surmounted by a large bronze cone still preserved within the area of the Vatican. Constantine, in his turn, saw nothing inconsistent with the Christian faith in the practice of commemorating members of the imperial house with aweinspiring splendour, though he naturally chose to link such sepulchres with a church. His own mausoleum, a rotunda placed alongside the Church of the Holy Apostles at Constantinople, has disappeared, but the circular mausoleum assigned to his mother Helena still exists, somewhat decayed, under the name of Tor Pignattara in the eastern suburbs of Rome.

A better idea of such buildings is, however, conveyed by the round church of S. Costanza in the northern outskirts of the city (fig. 58). This was in fact erected as a mausoleum for Constantina, the emperor's daughter, some twenty years after Constantine's death, but it reflects the style and pattern of earlier structures. S. Costanza adjoins the basilica of S. Agnese, but, whereas S. Agnese was to a large extent rebuilt in the seventh century, S. Costanza remains substantially in its original form.

58. Rome, S. Costanza: the outside, as shown in an engraving attributed to S. J. Neele, 1796

The outside is a circle of unadorned masonry, varied only by an entrance hall, 18 metres in breadth and flanked by two apses. Splendour alike of design and colour was reserved for the circle of the mausoleum itself. The entrance gives onto an ambulatory which leads all round the building. This passageway, topped with a barrel vault,has on one side the main external wall, into which have been recessed several niches of varying shape; on the inner side, it is divided from the high drum of the cupola by twelve pairs of granite columns (fig. 59).

59. Rome, S. Costanza: interior

The columns are all of identical pattern, being topped by richly-carved 'composite' capitals, in which the scrolls of the Ionic are added to the acanthus leaves of the Corinthian style. The capitals, in their turn, are surmounted by lofty imposts of stone which directly support the brickwork of the arcade. At the four cardinal points the arches are bigger than the rest, one of these enlarged arches marking the entrance while, directly opposite, another encloses Constantina's vast sarcophagus. Windows pierced in the drum of the cupola give a certain amount of light which would have been enhanced by the glow of lamps playing upon the brilliant decoration of mosaic.

Constantine, when carrying out his ambitious programme of church-building, was at least as vigorous in Palestine as at Rome, though in the Holy Land his martyria were designed to shelter a venerated place rather than a venerated relic. His mother Helena had visited Palestine as a pilgrim and established several shrines, including one over the grotto of the Nativity. In the words of Eusebius, the holy Empress, wishing to preserve with diligence the memory of Christ's infancy-, took care to give to the holy grotto a rich and varied decoration. Soon afterwards the Emperor himself, surpassing the splendour bestowed by his mother, embellished the same spot in a truly royal manner with the use of gold, silver and rich tapestry.

St Luke, in his Gospel, merely records that, after the birth of Jesus, they 'laid him in a manger' but, from the second century, the Cave of the Nativity was pointed out and there Constantine built his church, as a pilgrim from Bordeaux, who came to Bethlehem in the year 333, bears witness.