This baptistery has a quadrilobate plan, and a study of the structure has concluded that it was built in this form around the year 1025, although a plan of 1570 shows it as an octagonal building and a document of 1600 describes it thus: “Above the cemetery, to the north stands a small but complete chapel with eight corners covered by wide stones (slabs), and it is believed that in antiquity this was a temple of…”. That the memory of the existence of a pagan temple was still alive in 1600 indicates that, as in many other cases during the transition from paganism to Christianity, a pre-existing place of cult had been transformed and re-used according to the new faith.
This hypothesis is supported by the discovery (in 1574) of a reliquary shaped like a sarcophagus with acroteria. This contained minute fragments of a small box made of bone which must have been decorated with silver laminae with pagan motifs in relief, and which had been re-used as a reliquary. The reliquary and the laminae have been dated to the late antique period on the basis of their stylistic characteristics and it seems likely that they came from a cult building on the same site.
Excavation of the entire baptistery floor revealed a complex stratigraphy with four distinct phases and a baptismal font of unusual construction. It is faced with fine opus signinum, with eight vertical string-courses which divide the exterior wall to give an octagonal appearance. The bottom of the font is formed from a slab of granite with a central hole for the drainage of the baptismal water. The font is surrounded by a sort of annular bench (sedile) with a convex surface, made of rose-coloured opus signinum. (Laura Simone Zopfi, Paul Blockley)