The area of the surrounding piazza has always been a market square.

The Gothic building with its bicolored marble exterior is clearly tied to the Pistoian Romanesque tradition but it is known as "in corte" for its link with the Lombard curtis domini regis (the palace of the steward-governor of the city , which stood on the site in the 8th century. The area has always been a market square). The Baptistery was built in its present form just after the mid-fourteenth century. It is a central plan octagonal building. Lanfranco da Como made the font in 1226 (restored 1965 with Baroque additions removed).