As in all Gothic cathedrals, the architecture and decorative elements of the Duomo too were designed from the very beginning to convey a message of Christian salvation.
All art in medieval religious buildings was conceived as a means for teaching the faithful. All that mankind needed to know – the history of the world, from its creation to the end of time, the dogmas of religion, the teachings of the saints, the hierarchy of virtues, the variety of knowledge, of the arts and crafts – was illustrated and taught by means of paintings, stained glass windows, sculpture and architectural elements.
In the view of medieval men, furthermore, cathedrals were always taken to be a symbol of the celestial Jerusalem and sculpture in the Duomo, in particular, embodies this idea.

The statuary decorations (3400 statues plus over seven hundred figures in the marble high reliefs) enhancing the capitals, vaults, marble ornaments, facade, large windows and the 135 spires and pinnacles, provide an image of Paradise: patriarchs and prophets, martyrs and saints indicate our eternal destiny as people redeemed by the sacrifice of the Cross, guided towards heaven by the Virgin Mary whose statue stands on the highest point of the cathedral.
The statuary is a specific and at the same time pre-eminent aspect of the Duomo. Indeed, it has been produced without interruption for six centuries: it is the only on-going testimony of sculpted works of a cathedral construction site that has continued to produce statues, year after year, that are consistent with the cultural climate of each individual historical moment.

There were a number of devices that the sculptors had to comply with for such an impressive building: in particular, they had to bear in mind the need to look at the Cathedral from below, a need that caused the sculptors to emphasise, to differing degrees, the proportions of the various parts of each work, to increase or decrease the volume and plasticity of the forms, to adapt the bodies and their postures.
In the post war period, many sculptures, especially the most significant ones, have been moved to the Milan Cathedral Museum where they are preserved and shown to the best advantage as examples of this extraordinary artistic heritage.
In March 2006, the statues of the beatified Cardinal Ferrari and Father Monti were placed amongst the saints and the pinnacles of the Duomo and in October 2013 even the statue of the Blessed Don Carlo Gnocchi, thus continuing this unusual record of uninterrupted production of statues, spanning over six centuries, so much so that it has become the only collection of sculpture of its kind in the world.

The outer perimeter, the roof terraces and the 135 spires and pinnacles are covered with thousands and thousands of ornamental elements, each one different from the others. The richness of the interior decorations is concentrated in the very tall and original capitals topping the large polystyle pillars, in the large 16th century altars and in the monumental complex of the presbytery where the sculpture was carried out in various modes, using differing materials and techniques.

For further information:
E. Brivio, La scultura del Duomo di Milano, Milan, NED, 1982