Above the Old Town looms the Bischöfliche Hof (Bishop's Court), the old town fortress, which originated as a Late Roman castello. At the highest part of the town fortress stands the Cathedral whose architecture, except for the tower and two chapels, is completely Romanesque. At the time of the crusades the Langobard builders constructed an angular and archaic-looking basilica here. They created figurative capitals and almost lifesized apostle columns, whose bodies seem to grow directly out of the stone.

The cathedral is more than a Romanesque construction: it shelters wall paintings and altars from all the epochs, among them Switzerland's most richly ornamented Late Gothic altar. Following a long period of renovation, the cathedral was reopened in autumn 2007. In the cathedral treasury, Late Roman and medieval ritual objects made of gold, silver and ivory illustrate the 1600-year-old tradition of the oldest still functioning bishopric north of the Alps.