The buildings which surrounded the public squares corresponded in lavish magnificence to the altars, statues, dedicatory columns, and triumphal arches. Broad colonnades with shops formed the enclosure, interrupted by temples, and courts of justice, or curias, which can have differed but little in external appearance from the sacred edifices. Most important among these public buildings were the basilicas, which, in name, purpose, and form, were derived from Greek prototypes. As halls of justice and places for commercial traffic, they may be regarded as covered extensions of the open squares. Several of these buildings, erected during the imperial epoch, are known by considerable remains, but they deviate so greatly in disposition as to have no plan in common beyond that of a hall surrounded by narrow aisles. The oldest Roman structure of this kind, the Basilica Porcia built by Cato in 185 B.C., was of an oblong shape, abutting with one of its ends upon the Forum, while the other was enlarged by a small exedra, or apse. (Figs. 287 and 288.) The chief space was surrounded upon all four sides by two-storied aisles, the central hall, however, not rising above them, as in the Christian basilica, this being difficult of construction because of the slightness of the shafts, and not necessary for the introduction of light. A portico with a flat roof was erected above the entrance, enlivening the bare and extended front wall. Thus the Basilica Porcia did not differ in principle from the early Christian church, and the similarity appears also in the other basilicas of the Roman republic, all of which had their front upon the smaller side. In the courts of the imperial epoch, however, this primitive type was treated with great freedom, and nothing remained of the original arrangement but a large central hall surrounded by a double passage of arcades upon piers, without columns and without an apse. The normal basilica, described by Vitruvius, with two-storied side aisles, faced with its greatest length upon the public square, and had an apse; the basilica at Fanum, built by the Roman writer, was similarly arranged upon the facade, but a clere-story supported upon gigantic columns rose above the lateral passages. These passages opened, from the end opposite the entrance, into an adjoining temple, the pronaos of which served as the tribune of the forensic court. The basilica at Pompeii, of which the narrow side was the front, had no apse, while the Basilica Ulpia had great exedras upon both ends, with the entrance portal upon the longer side. The Basilica of Maxentius (Fig. 289), which was completed by Constantine, was an exception in every respect, being entirely vaulted, and having two apses upon adjoining sides opposite to the two chief entrances. The whole formed one of the most remarkable and important halls of antiquity, with the consideration of which the history of Roman architecture may well be terminated. The original type of the basilica was wholly neglected by later architects, who treated the problem of a forensic hall without restrictions, utilizing the accidental formations of the ground, while endeavoring to combine suitability and the display of ingenious constructions with magnificent novelties of their own invention.
The Roman dwelling-house was, in the earliest ages, identical with that of Etruria, and, indeed, of all Central Italy. Although related to Hellenic prototypes, the peculiarly Italian atrium, without columnar supports for the roof, remained in use even after the general introduction of the Greek peristyle. At Pompeii a combination of these two varieties of court is met with, the front space being a simple atrium, and that further within a peristyle. Each enclosure was surrounded by chambers. (Figs. 290 and 291.) The mosaic and painted decoration of the floors and walls will be treated in a later section. The small chambers were lighted only through doors opening from the inner courts, and did not share in the architectural importance assigned to the larger halls, which, in the last years of the republic and in the imperial period, transformed the houses of the wealthy into veritable palaces. With the luxury of the table, the magnificence of the dining-room was increased; and, with the growing taste for literature and art, extensive libraries and galleries of pictures became prominent features. Many of the forms adopted for this palatial architecture appear to have been derived from the later Greeks; the designation of halls, as those of Egypt and of Kyzicos, employed by Vitruvius, pointing to the sovereignties of the Diadochi. This enlargement of extensive rooms by columns was, however, in a great degree supplanted by vaulting, in which case the columns were introduced merely as decorative members. Much attention was devoted to a lavish enrichment of these rooms, the shafts being colored marble monoliths, the lacunæ of the vaulted ceilings overlaid with bronze or richly gilded, and the capitals being sometimes formed of solid metal. One of the halls in these palatial residences, the private basilica, though it may not have been universal, deserves especial consideration because of its great importance in later times. Such courts of justice are mentioned by writers of the Augustan age as forming part of the dwellings of men of condition, “because in their houses councils were held upon public and private matters, and civil cases decided.” These halls were naturally modelled in a great degree after the public basilicas upon the forums, such as the Porcian, Æmilian, Sempronian, and Opimian basilicas, which had been built during the republic; but they appear, when compared with the primitive type of the Roman basilica, to have differed fundamentally in two respects. In the first place, the hall, being surrounded by the chambers of the dwelling, could not be provided with windows like the free-standing, forensic basilicas, and a clerestory rising above the adjoining rooms was consequently adopted. This rendered necessary a second modification. To impose a heavy wall of masonry, besides the timbered ceiling and roof, upon a double story of columns must have seemed inadmissible to the Roman taste for substantial construction. The aisles upon the front and rear were consequently given up, the columns and galleries remaining upon the sides only, the massive masonry of the enclosure thus receiving the thrust of the clere-story wall, and greatly increasing its stability. (Fig. 292.) This loss of continuity could have been of no great disadvantage in the private basilica, as it did not serve, like the free-standing public structures, for traffic and promenades, as well as for sessions of justice. The galleries over the side aisles were frequently omitted, and it appears to have been in these halls that the connection of columns by arches, in the place of lintels, was first introduced. Such archivolts are first known by examples built during the reign of Diocletian, as at Spalatro (Fig. 293); but they soon came into general usage, their practical advantages outweighing the want of æsthetic fitness inherent in such curved entablatures. It was from these private basilicas that the first Christian churches were architecturally developed. The believers had assembled, during the imperial ages, in the houses of wealthy converts; and as these halls of justice had been used for religious services during times of persecution, it is not strange that, after the recognition of Christianity by the Roman government, their arrangement and even their name should have been retained.