Overshadowing even the cathedral, the octagonal pink-marble baptistery on the south side of the piazza is one of the most important such structures in Italy. Its architecture is a hybrid of Romanesque and Gothic, and its construction started in 1196 on the cusp of the two great architectural eras. The interior is particularly stunning, with its interplay of pencil-thin marble columns and richly coloured 13th-century frescoes in the Byzantine style, interspersed at irregular intervals with statues and bas-reliefs.
Architect and sculptor Benedetto Antelami oversaw the project and it contains his best work, including a celebrated set of figures representing the months, seasons and signs of the zodiac. The baptistery wasn't completed until 1307 thanks to several interruptions, most notably when the supply of pink Verona marble ran out.