On the left of the cathedral, from a door at the base of the bell tower, you enter the
church of San Giovanni, also called the “crypt”. Inside it it’s preserved the baptismal font designed by Rossellino and a collection of valuable illuminated pages.
In the crypt there are also the remains of the sculptural decorations of the ancient medieval church of Santa Maria and a series of three Flemish tapestries. From here you enter the so-called “cathedral labyrinth”, a system of drainage tunnels dug under the apse of the cathedral to stem the problems of collapse of the structure above, which occurred as early as the fifteenth century.