Every medium-sized city in Italy has one (and only 1) Duomo, serving as the bishop's seat (a.k.a. cathederal.) Verona's present day version was started in 1139 and is a nice example of Romanesque architecture. As with most Romanesque edifices, the stone carvings are lively and worth closer inspection.
While the interior is plain compared with San Zeno or Sant'Anastasia, it has a certain austere charm. Many just come to see Titian's Assumption (not as famous as the one in Venice, but still lovely) and leave. They miss the entrance at the far left to the cloister.
Here one can find the excavations of earlier religious buildings on this site. There was an 8th century church, S. Giovanni in Fonte, which in turn was built with recycled Roman masonry. As one might expect from a church dedicated to St. John the Baptist, there's an impressive early christian baptismal font.
The Duomo does not get many visitors, and you are likely to have the excavations completely to yourself. There is a nominal entrance fee for tourists who are not attending mass.