Like the basilica, the baptistery was founded by Constantine in the 4C. Today it is used for ceremonies on Holy Saturday. In the 5C, it was rebuilt by Sixtus III who erected the eight porphyry columns in the centre and inscribed the words relating to baptism on the octagonal entablature. The upper colonnade and the lantern date from the 16C. Chapels were added to the baptistery by different popes over the years and Urban VIII gave the building its present appearance in the 17C; the frescoes date from this period.