Romanesque architecture was so designated because its forms and structures harkened back to Roman models of six centuries earlier. But this was not a conscious return to the past, but a continuous development from the first years of Roman Imperial Christianity.
In the early part of the fourth century, the Christian population in Europe was subject to persecution from the imperial government of Rome. Because of this, churches were not a large part of the worship. The faithful met in homes and catacombs where their saints and martyrs were buried. In 313, the Emperor Constantine issued the Edict of Milan, which legitimized Christian worship. Constantine because a Christian himself and became a patron of the Church and church buildings. With imperial support, an architecture that reflected imperial support began to develop. Two Roman structures were used in these early Roman churches – the basilica and the “centrally-planned” church (Hagia Sofia, Pantheon, etc.)
This article is about the basilica which eventually became the predominant model for Christian churches. The Latin basilica (derived from Greek, βασιλική στοά, Royal Stoa, the tribunal chamber of a king) was a Roman public building and could be used as a legal court, a market, or even a training ground for soldiers.
Such buildings usually contained interior colonnades that divided the space, giving aisles or arcaded spaces on one or both sides, with an apse at one end (or less often at each end), where the magistrates sat, often on a slightly raised dais. The central aisle tended to be wide and was higher than the flanking aisles, so that light could penetrate through the clerestory windows. They were usually wooden-roofed structures.
After the Roman Empire became officially Christian, the term “basilica” came to refer specifically to a large and important church that has been given special ceremonial rites by the Pope. Thus the word retains two senses, one architectural and the other ecclesiastical.
It is easy to see how this Roman basilica turned into the Romanesque church that we know so well. With the addition of a stone vault, a crossing tower, transepts, ambulatory and apsidal chapels in echelon this could be any number of medieval structures.
Technical note: I have recently discovered the Google “Sketch Up” 3-dimensional modeling application. This free software is used to model many of the buildings on Google Earth. With no personal modeling experience at all, I have been able to do rudimentary 3-D models of churches to illustrate the architectural concepts developed in this post. “Sketch-Up” is an amazing tool and part of the enormous resources that the web has put in our hands.