The Baptistery of San Giovanni was founded on 15 August 1152. It is here that the Sacrament of Baptism is administered and the Christian embarks upon the path of Faith. The reason that such a fascinating and enigmatic building was constructed was certainly the wish to endow the cathedral with a worthy adjunct: a Baptistery that, in terms of position, size, materials and style, would be in harmony with the majestic building that already stood opposite.
https://www.opapisa.it/en/square-of-miracles/baptistery/