The earliest massive church buildings, such as the Cathedral of San Giovanni in Laterano in Rome, consisted of a single-ended basilica with aspidal finish and a courtyard, or atrium, at the other finish. As Christian liturgy developed, processions became part of the proceedings. The processional door was that which led from the furthest finish of the building, while the door most used by the public might be that central to side of the building, as in a basilica of law. This is the case in lots of cathedrals and church buildings.
As numbers of clergy increased, the small apse which contained the altar, or table on which the sacramental bread and wine were offered in the rite of Holy Communion, was not sufficient to accommodate them. A raised dais called a bema formed part of plenty of gigantic basilican church buildings. In the case of St. Peter's Basilica and San Paolo fuori le Mura (St Paul's outside the Walls) in Rome, this bema extended laterally beyond the main meeting hall, forming arms so that the building took on the shape of a T with a projecting apse. From this beginning, the plan of the church developed in to the so-called Latin Cross which is the shape of most Western Cathedrals and gigantic church buildings. The arms of the cross are called the transept.
One of the influences on church architecture was the mausoleum. The mausoleum of a noble Roman was a square or circular domed structure which housed a sarcophagus. The Emperor Constantine built for his daughter Costanza a mausoleum which has a circular central space surrounded by a lower ambulatory or passageway separated by a colonnade. Santa Costanza's burial place became a place of worship & a tomb. It is of the earliest church buildings that was centrally, than longitudinally planned. Constantine was also responsible for the building of the circular, mausoleum-like Church of the Holy Sepulchre in Jerusalem, which in turn influenced the plan of a few buildings, including that constructed in Rome to house the remains of the proto-martyr Stephen, San Stefano Rotondo & the Basilica of San Vitale in Ravenna.
Ancient circular or polygonal church buildings are comparatively rare. A small number, such as the Temple Church, London were built in the work of the Crusades in imitation of the Church of the Holy Sepulchre as isolated examples in England, Germany & Germany. In Denmark such church buildings in the Romanesque style are much more numerous. In parts of Eastern Europe there's also round tower-like church buildings of the Romanesque period but they are usually vernacular architecture & of small scale. Others, like St Martin's Rotunda at Vishegrad, in the Czech Republic, are finely detailed.
Other than Santa Costanza and San Stefano, there was another significant place of worship in Rome that was also circular, the giant Ancient Roman Pantheon, with its numerous statue-filled niches. This was to become a Christian church and lend its style to the development of Cathedral architecture.
The circular or polygonal form lent itself to those buildings within church complexes that perform a function in which it is desirable for people to stand, or sit around, with a centralised focus, than an axial. In Spain the circular or polygonal form was used throughout the medieval period for baptisteries, while in England it was adapted for chapter houses. In Spain the aisled polygonal plan was adapted as the eastern terminal and in Germany the same form is often used as a chapel.
