Baptistery

 

The baptistery with its distinctive octagonal shape is the oldest of Christianity and was the master for all following baptisteries. It was built in 315 a.C. by Constantine, presumably in a round shape, and was rebuilt octagonal by Sixtus III between 432 and 440. Despite many renovations parts of the antique mosaic and pillars of egyptian porphyr are still visible today. Even a bronze door from the 5th century could be saved; it is the last of its kind from Ancient Rome.