The Cathédrale Saint-Pierre stands at the heart of the old town, its facade rising above a small cobbled square off the main street of the old city. Construction began in the thirteenth century and continued, in fits and starts, for over six hundred years — the result is a building that reads like an architectural timeline, with Romanesque, flamboyant Gothic, and nineteenth-century additions sitting side by side.
Inside, the most remarkable feature is the tomb of Saint Vincent Ferrier, a Dominican preacher who died in Vannes in 1419 during a preaching tour and was later canonised. The treasury contains several fine pieces of medieval goldsmithing. The cathedral is free to enter and worth a look even for visitors who don't usually seek out churches — the interior is cool, dark, and genuinely atmospheric.