Final answer:
In early Christian churches that were adapted from the Roman basilica, the altar was typically placed within the apse at the end of the central nave. The addition of a transept gave these buildings a cruciform shape to signify Christian significance.
Step-by-step explanation:
In the adaptation of the Roman basilica for Christian public worship, the altar was placed at the end of the central nave within the apse. The basilica layout was beneficial due to its large interior space suited for housing congregations, an important consideration given the communal nature of Christian worship which emphasized gathering, prayer, and rituals like baptism.
The transept was added between the nave and the apse, running perpendicular to the nave, which gave the building a cruciform shape—symbolic of the crucifixion of Jesus Christ. This architectural choice highlighted the interior over the exterior and was crucial to conveying the new religious authority of Christianity through architecture. One example of such a basilica is Old St. Peter's in Rome, which followed the Roman basilica plan with a crucial addition of a transept to attain this significant symbol.