Italy’s oldest baptistery, dating from the 4^th century. It is located next to the Basilica of St John Lateran. Many legends are attached to this, particularly relating to Emperor Constantine, ranging from him being responsible for its building to him actually being baptised there, which is unlikely. It is said to have been inaugurated between 320 and 325, upon the site of a Roman spa, as a round building. This is perfectly possible, but a more likely story is that it was built during the papacy of Sixtus III (432-440) in an octagonal shape, for reasons discussed on the previous page. Originally entry was through a grand portico, but that is now closed and entry is through a rather ordinary looking door on the north side.
  The baptismal font was originally designed for total immersion: what is there now is a smaller replacement from c 1518, surrounded by a balustrade. Around this are eight porphyry columns, said to have been donated by Constantine.  
  Much has changed over the years. The original dome was beyond repair, and was replaced in 1540: tragically, some fine mosaics were lost.  The baptistery was much embellished by Pope Urbanus VIII (1623 to1644.) and some, including myself, would say not for the better, though the marble floor by Borromini is very good. The dome and surrounding walls were frescoed with scenes of the life of Constantine  by a team of second rank artists.