Lateran Baptistery (church building) built
400 CE.
Shown on the map at 41.886°, 12.504°.
The Lateran Baptistery (Italian: Battistero lateranense, also known as San Giovanni in Fonte or San Giovanni in Onda) stands apart from the Archbasilica of Saint John Lateran, Rome, to which it has become joined by later construction. This baptistery was founded by Pope Sixtus III in 440, perhaps on an earlier structure, for a legend grew up that Constantine the Great had been baptized there and enriched the structure. However, it is more likely that if he was baptized it was in the Eastern part of the Roman Empire and possibly by an Arian bishop. This baptistry was for many generations the only baptistery in Rome, and its domed octagonal structure, centered upon the large octagonal basin for full immersions, provided a model for others throughout Italy, and even an iconic motif of illuminated manuscripts, "The fountain of Life". Around the central area, where is the basin of the font, an octagon is formed by eight porphyry columns, with marble Corinthian capitals and entablature of classical form. On the ceiling of the Baptistry is the story of the Battle of the Milvian Bridge (312 AD). An ambulatory surrounds the font and outer walls form a larger octagon.
Text from the Wikipedia article Lateran Baptistery, licensed CC BY-SA 4.0.