Built over a period of 250 years following designs by Diotisalvi, this majestic circular building with a circumference of 110m is covered with an unusual dome crowned with a small truncated pyramid which is in turn topped by a statue of St John the Baptist. The interior houses a fine octagonal font sculpted in 1246 by Guido Bigarelli, as well as an admirable pulpit sculpted in 1260 by Nicola Pisano.