Aosta’s cathedral has ancient origins, associated with the initial stages of the diffusion of Christianity in the region. It was founded near the old Roman forum complex at the end of the 4th century, when the diocese was probably established. Rebuilding commenced at the end of the 10th century and was completed around 1065, several decades later. Much of the Romanesque building can still be seen: the basilica plan with nave and two aisles, the crypt, the two eastern bell towers and the great square piers. However, the Romanesque church ended in a massive semicircular apse on the western side, flanked by two bell towers mirroring those on the eastern side. As in the Early Christian building, the entrance was on the southern side. The great rebuilding of the 11th century involved important craftsmen and painters, who decorated the walls of the nave. Part of the upper register of the complex fresco cycle can still be seen in the attic today: a frieze with arches with the figures of the forefathers of Christ and the legend of Saint Eustace on the northern wall, and portraits of ecclesiastic personages and Old and New Testament scenes on the southern one. The mosaics adorning the floors and the ambulatory, with radial chapels that replaced the eastern Romanesque apses, are among the most important changes and embellishments made in the 12th and 13th centuries, which culminated in the demolition of the massive western structure, the construction of the ribbed cross vaults and the erection of a new façade, between 1522 and 1526, whose decoration with frescoes and terracotta sculptures was entrusted to the Lombard painter Ambrogio Bellazzi da Vigevano. The new marble high altar, by Lugano sculptor Francesco Albertolli, was installed in the mid-18th century (1758), while the Neoclassical façade designed by the architect Gayo (1846–48) and the Neogothic chapel dedicated to the Holy Rosary (1862) are dated a century later.