The interior of the cathedral presents the different styles then existing in Lombardy from baroque to rococo. Extremely interesting is the Tree of the Cross, magnificent fresco on the south wall of the transept, attributed to Arcimboldo (1556). Very important, on the left side of the nave, is also the fourteenth century 'evangelicatorio' made by Matteo da Campione, with figures of apostles and evangelists. Between the chancel and the choir stands the neoclassical altar made by Appiani and the main altar, a masterpiece made in the fourteenth-century by Borgino del Pozzo, with embossed gold leaf and scenes of John the Baptist. The frescoes, executed by Zavattari, constitute a true masterpiece of the International Gothic style. Extraordinary breadth and completeness, the figure celebrates the cycle of Teodolinda, winding in forty-five scenes. In the Chapel of Teodolinda retains the 'Crown of Iron', consisting of six rectangular plates of gold connected by hinges and a metal inner ring that wants to be a tradition derived from one of the nails of the Passion of Christ. Each plate is decorated with gems, gold and cloisonné enamels rosettes.