The Cathedral of Aosta presents a curious face to the visitor: the white neoclassical facade on Piazza Giovanni XXIII dates from 1836 and gives little indication of what lies inside. The interior is largely Gothic, built over earlier Romanesque and early Christian structures, and the sequence of layers is part of what makes the Cathedral worth extended attention rather than a quick glance.
The choir contains two exceptional items: a set of 15th-century carved wooden choir stalls comparable to those at Sant'Orso, and a pair of mosaic pavements from the 11th century, still intact in the floor of the nave, depicting the calendar months and a cosmological map of the known world surrounded by allegorical figures. The treasury (Museo del Tesoro, accessed separately) holds one of the best collections of religious metalwork, ivories, and embroidered vestments in the Alpine region, accumulated over fifteen centuries of continuous use. The excavated area beneath the Cathedral is open to visitors and shows Roman walls and floors that lie below the present building — clear evidence of the continuous human occupation of this site since antiquity.