Dedicated to Saint Tudwal (died ca. 564), first Bishop of Tréguier. Former cathedral, it was the seat of the Bishopric of Tréguier, abolished under the Concordat of 1801, when its territories were divided between the Diocese of Quimper and the Diocese of Saint-Brieuc, known since 1852 as Saint-Brieuc-Tréguier. The building is mostly Gothic, 14th and 15th centuries with one remaining north tower (called Hasting Tower) from an earlier Romanesque church, ca. 1090-1110. It is unusual that all three towers are part of the transept. The nave has three aisles and terminates in an ambulatory with three radiating chapels. There is an attached cloister, built in 1468.