Notes: This domed building started construction in 1152, and was finally completed in 1363. This structure replaced an older baptistry.

The architect Diotisalvi, left a signature that can be read on two pillars inside the building, with the date 1153:

MCLIII, MENSE AUGUSTI FUNDATA FUIT HAEC ECCLESIA
DEUSTESALVET MAGISTER HUIUS OPERIS

The Baptistry has a circumference of 107.24 m and is 54.86 m high. An interesting aspect of the architecture is that it represents the transition from the Romanesque style to the Gothic style. The upper registers are in the Gothic style, with pointed arches and the lower registers are in the Romanesque style, with rounded arches. The material used is Marble.