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San Giovanni in Fonte (also known as the Lateran Baptistery) is an early Christian baptistery located next to St. John Lateran and the Lateran Palace in Rome. Built by Constantine the Great in 315, the ancient structure is one of the oldest Christian buildings in Rome and the oldest baptistery in all of Christendom. It provided the model for later versions, including the famous Byzantine baptisteries at Ravenna.

Interior of Lateran Baptistery. Rome, Italy

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 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. An ambulatory surrounds the font and outer walls form a larger octagon. Attached to one side, towards the Lateran basilica, is a fine porch with two porphyry columns and richly carved capitals, bases and entablatures.

Interior of Lateran Baptistery. Rome, Italy

For centuries the main entrance to the Baptistery was from the opposite side: it was preceded by a Vth century small portico with two porphyry columns. In 1575, in the frame of the redesign of the whole Lateran area, Pope Gregory XIII opened an entrance from the square; it was the beginning of a series of modifications/restorations which lasted for almost a century until they were completed by Pope Alexander VII.

source and read more
http://romeartlover.tripod.com/Vasi101.html#Battistero
https://en.wikipedia.org/wiki/Lateran_Baptistery
http://www.sacred-destinations.com/italy/rome-lateran-baptistery