The early Christian architecture began in Rome and Constantinople in the 3rd and 4th centuries AD. As Christianity spread and was adopted as the state religion by Emperor Constantine, new church buildings were constructed to serve the needs of worship. The most common design was the basilica, adapted from Roman civic buildings due to its suitable layout of a central nave flanked by aisles. Examples like St. John Lateran and St. Peter's in Rome illustrate this basilica design. Some alternative centralized and circular plans were also occasionally used, such as mausoleum churches like St. Constanza in Rome.