Built over a period of more than two centuries following a project of Diotisalvi, this circular building merges Romanesque elements which have an echo in the cathedral with early Gothic influences. Inside, as well as the vocal performances of the attendants who will prove its exceptional acoustics, there is an attractive octagonal baptismal tank by Guido Bigarelli in 1246, and an admirable pulpit sculpted in 1260 by Nicola Pisano.