The builder of the baptistery is disputed with some sources suggesting the Emperor Constantine while others credit it St. Severus, bishop of Naples from A.D. 363-409 -- a friend and associate of St. Ambrose of Milan.
The baptistery itself measures seven metres by seven (23 feet x 23) and is octagonal in shape. The baptismal pool itself it a shallow pool which would not have permitted full immersion. Instead the candidate and sacred minister would have stood in the pool and water would have been poured over their head -- similar in many regards to how baptisms today tend to be conducted.
What is particularly noteworthy in the baptistery are the extant mosaics found on the ceiling which are probably from the fifth century -- with its characteristic use of blue glass from this period in Italy. They are quite striking. At the pinnacle of the domed ceiling is the Chi Rho and the Greek Alpha and Omega which, of course, makes reference to Christ as the beginning and the end. A hand emerging from the heavens, that of God the Father, is shown holding a laurel of victory.