Built out of granite and continuously modified by adding new structures, the cathedral is an extremely composite building.

In this period the nave and the ornate gateway at the northern end of the north transept – whose twelve niches, according to Breton custom, were supposed to accommodate the Apostles – were built high.

Outside, in front of the central pillar of the large gate, stands a statue of the Dominican friar St. Vincent Ferrer, from Valencia.

The northern façade opens onto the garden of the cloister (ruins from the 16th century) and the Rue des chanoines ("Street of the Canons") through the beautiful portal at the top of the north transept, built in a Flamboyant late Gothic style (1514), and decorated with twelve niches designed to house statues of the twelve apostles.

In the novels of Alexandre Dumas, the musketeer Aramis appears at one point as Bishop of Vannes and logically must have served in this very cathedral.