The differences and similarities from the design of the Early Christian church to the Romanesque church are defining in the fact that one is based upon the other. The Early Christian churches were mostly basilicas, which was then modified and became the cruciform structure of the Romanesque churches.
An Early Christian church has some basic components to it. There is the nave that is the center aisle that leads to the apse. The apse is a semicircular projection from the back of the structure opposite the entrance. Running along the sides of the nave are side aisles.
Basilicas are long rectangular buildings with a higher ceiling over the apse than the side aisles. At the top of the nave wall at either side were commonly a row of windows called the clerestory to allow in natural light.Order now
Many basilicas from the time were built by poor Christians, so examples like Saint Sabina used recycled columns in the construction. The floor was probably originally made with packed dirt and there would have been no seating. Old Saint Peter’s basilica was probably built with money donated by Constantine. It wasn’t until later, when Christians became accepted and wealthy that basilicas like Sant’ Apollinare were built with luxurious items such as gold leaf backed glass in mosaics for extravagant decoration to heighten spirituality within the building. The outsides of these buildings, even with this new wealth and acceptance remained plan to enhance the idea of the separation of the earth world and the spiritual world.
Romanesque churches are a modified version of this basic format. The staggered roofing is still the same with the highest ceiling above the nave. Because of a rise in pilgrimages, rather than just having a side aisle that runs along the sides of the nave there was an ambulatory that was added that runs along the whole interior perimeter of the building. This way, pilgrims who come don’t have to wait for mass to be over to see relics and pray to the saints and when they go about their business they will not disturb mass either.
Another add on is the extension of the space in front of the nave, before the apse, this area was extended outward to form the distinct cross floor plan of the Romanesque churches. This space was for overflow attendance in mass and was not the only accommodation for this overflow. Another option for a possible flux in attendance would be to sit in the gallery which is a second floor hallway that was added on top of the side aisles.
Like Early Christian churches there are columns within these churches and also like them, they are connected by arches creating an arcade, though his arcade served a design purpose as well as a structural purpose in the Romanesque church. There was no standard of measurement so the space between each column was a bay and the churches were built apse to entrance, bay by bay. But if you look at two churches from this time such as Saint Sernin and Saint Etienne it is easy to see that this bay measurement varied from structure to structure.
This manner of building would have been funded by the city that surrounded them, so if the city ran out of money the construction was out on pause. The reason I point this out is because another structural difference between Early Christian and Romanesque churches are the buttress, a means of extra support for more height. If you look at Saint Sernin there are buttress around the front entrance but no added height to support that is because two towers like those around Saint Etienne’s entrance were planned but construction was never un-paused to complete them. But even with the added buttresses and towers, when they could be afforded, the exteriors remained plan with a more extravagant interior for the similar differentiation of spiritual and real worlds.
Overall the Early Christian basilica and the Romanesque cruciform are very similar. The cruciform is just a modification of the basilica as Romans became more familiar and comfortable with their stone work and architecture. With the on set of the Gothic style the same thing happens, the similar structure is modified to fit the beliefs, aesthetics, and ability of the time and place.