The duomo of San Tommaso di Canterbury sits at the head of Piazza della Repubblica, the centre of the old town. Begun in the Norman period and given its present pale baroque facade in the 18th century, it is the largest church in Marsala and the seat of the diocese of Mazara del Vallo.
The interior is pleasantly cool and uncluttered, with three naves and a few good 16th-century paintings by the Trapanese school. The treasury behind the cathedral, in a separate building, holds an eight-piece cycle of Flemish tapestries from the 1530s — Spanish royal gifts — that depict scenes from the capture of Jerusalem and are among the finest in Sicily.