The façade, characterized by alternating black and white marble bands, measures 17.30 m in height and 18 m in width. It has a tiered pattern and a salient scheme and is tripartite following the internal subdivision of the naves. The division is made with pilasters connected to each other at the top thanks to the succession of small hanging arches typical of Romanesque architecture. At the base there are three openings surrounded by small beam columns and surmounted by pointed lunettes, once frescoed. The paintings on the minor doors have not survived, while the one in the central lunette is now preserved inside the church and portrays the Virgin enthroned with a royal mantle and crown holding the holy child with the Saints Nazarius and Celsus at her sides. Above the entrances there are three circular windows: the lateral ones of modest size, are surrounded by white and black ashlars, the central one, however, turns out to be a real rose window, surrounded by valuable terracotta painted green. The innermost ring is made up of intertwined Gothic arches followed by four rings with floral decorations separated by strips in the shape of spirals, ribbons and cords. In the center the stained-glass window represents the patron saints Nazaro and Celso. Between the latter and the main portal is a Gothic aedicule containing a high relief of St. Ambrose, patron saint of the diocese.
Other friezes are found on the keystones of the entrances on which, starting from the left, we find a blessing hand, a lamb with a Greek cross and a Latin cross. Above the small round window on the right there is a stem of lake grass, also recurring in some Renaissance paintings. Finally, on the central pilasters, there are the Visconti family coats of arms: a snake flanked by the letters I O, and that of the Torriani family: a tower flanked by the letters A N.