The Cathedral of Sant'Antonio Abate clings to the cliff edge of the old town, just below the castle and above a sheer drop to the sea. The building dates from the late medieval period and has been modified many times, but retains a Romanesque-Gothic character in its stonework and proportions.
The interior holds the altarpiece attributed to the Maestro di Castelsardo — a 15th-century painting of exceptional quality for a provincial church, displaying the kind of stylistic sophistication that suggests an artist trained in Catalonia or Flanders. The crypt below the main floor holds the tombs of the Doria family, the medieval lords who built the castle above.