San Gimignano's main church — technically a collegiate rather than a cathedral, since the town never had its own bishop — contains some of the most complete cycles of 14th-century frescoes anywhere in Italy. The plain Romanesque façade gives no hint of what's inside. The entire nave is covered: the left wall with scenes from the Old Testament by Bartolo di Fredi (1367), the right wall with scenes from the New Testament by Barna da Siena, and the counter-façade with a terrifying Last Judgement by Taddeo di Bartolo. The effect is overwhelming.
Admission is charged, which keeps the crowd somewhat manageable. Allow at least an hour to take in the nave properly, and don't miss the Chapel of Santa Fina at the end of the right aisle — Ghirlandaio's work there is worth the separate look.