The French writer Ernest Renan (who was born in Tréguier) said the cathedral is a "master-piece of lightness". The oldest part of the church dates back to the 12th century: it's the Romanesque tower, called Hastings' tower. The other part, Gothic style, dates back to the 14th century.
The English, during the Hundred Years War, then the Spanish in 1592, Catholics and Protestants in 1594, destroyed the city and the cathedral. During the French Revolution, they melted the bells called Yves and Tugdual in order to make cannons.
Saint Yves is the saint who protects lawyers and he's the one who protect Brittany: he was buried in this cathedral! The duke Jean V gave the reliquary for Yves' body. The duke was taken prisoner during the Succession war of Brittany, and he prayed a lot saint Yves to get him out of the trouble!
He was set free, and thanked Yves by raising his tomb in the cathedral between 1420 and 1442. Then Jean V was buried next to Yves in 1451!