Answer: Some 25 of the original parish churches, or tituli, the first legal churches in Rome, still function. Most had been private houses in which the Christians illegally congregated, and some of these houses, as at Santi Giovanni e Paolo, are still preserved underneath the present church buildings. Since the 4th century the tituli priests have been cardinals who, over the centuries, have rebuilt, enlarged, and embellished their churches. Some early Christian churches were centrally rather than longitudinally organized, a plan dictated by the circular form of the imperial mausoleums into which they were built. A good example is Santa Costanza (c. 320 CE), which also has a superb series of 4th-century vault mosaics in pagan designs. Although churches of this type were few, they had a strong influence on the development of the centrally planned house of worship. However, it was the rectangular Roman basilica (a word used to designate a public hall in pre-Christian Rome and, later, an important church), with its open hall extending from end to end, that established the model for Western ecclesiastical architecture for centuries to come. The basilical church has a nave higher than the side aisles, from which it is separated by a colonnade on each side. It has either a cloistered court (atrium) or anteroom (narthex) or both at the west end and a semicircular projection (apse) at the east. In the 4th century CE Constantine I added the transept, a lateral aisle crossing the nave just before the apse, to the standard basilican plan, thus making the basilica a cross-shaped structure.
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