He was ordained as a priest and in 525 left Wales for the continent with a dozen companions. He landed at the Ile d'Ouessant and preached along the north Finistère coast up to the Île de Batz.
The cathedral is 80 metres long in total, 16-metre height under the vaults and 44 metres wide with the transepts.
In the north tower, there are three bells which date from more than three centuries ago, including the oldest bourden bell in Brittany, which weights more than 2 tonnes, and was cast in 1563.
It also has an ensemble which is almost unique in Brittany The 50-metre spires are from the end of the 14th-century.
In the 16th-century, side chapels gave it its definitive stature.
According to legend, he helped free the Île de Batz from a dragon which was terrorising the local population.
The cathedral replaced an earlier romanesque basilica founded by Bishop Haman and was built in various stages. It is dedicated to its 6th-century founder, the first bishop Saint Paul Aurelian. It bears the de Kersauzon coat of arms and the inscription: "HIC JACET IN PACE GUILLELMUS DE KERSAUZON EPS LEON. He was originally from Wales and is considered to have been the first bishop of the Léon area. Above the central door is a terrace which enabled the bishop to address a crowd and give blessings and under the right-hand tower is a small door used in the Middle Ages as an entrance for lepers Not as deep as the usual porch, the south porch is decorated with statues of John the Evangelist and saint Simon carved by the Yves Hernot workshop in the 19th-century on one side and saint Andrew and saint Peter carved by Guy Pavec in 1980 on the other. It leads to double doors which give access to the cathedral. Monseigneur Validire was the Bishop of Léon from 1427 to 1432.