The Cathedral of Notre-Dame du Glarier stands at the edge of the old town below the two hilltop fortresses, its Romanesque tower one of the oldest surviving structures in Sion. The current building is a mix of Romanesque and Gothic work, with a separate bell tower dating from the 11th century — the oldest part of the cathedral complex.
The interior holds carved choir stalls, late-medieval stonework, and the atmosphere of a working cathedral rather than a museum. The treasury contains ecclesiastical objects from the long history of the Diocese of Sion, one of the oldest Catholic dioceses in Switzerland. The cathedral is the centrepiece of the old town and worth visiting before or after the hill climbs.