The large dome that surmounts the baptistery is supported by four pillars inserted at the corners of the central span. The frescoes that cover it depict Christ Pantocrator in the central medallion, surrounded by the disciples arranged in a radial pattern. Christ has His head turned towards the altar, holding the book of the Word in His left hand, while His right hand is open and raised in the direction of the apostles, calling them to spread the Good News to humanity.
The geometric simplification of the forms that construct the face of Christ gives it a pronounced expressiveness: the elongated face, the pointed beard, the straight nose, the wide and open eyes, and the pronounced brushstrokes that define the eyebrow arches. The twelve apostles, also depicted with a certain hieratic quality in their poses, are slender and elongated figures, draped in cloaks with wide brushstrokes that create deep folds in the drapery. Similar to Christ, their expressiveness is primarily conveyed through their wide-open eyes and geometric simplification. Not all the apostles are identifiable today, but some, such as Filippo, Pietro and Taddeo, are still recognisable thanks to the presence of their names, even if incomplete, written in white chalk next to each figure.
Outside the circular space of the dome, on the drum below and in direct communication with the faithful, we can see the image of Mary holding the Infant Jesus in her arms. The Virgin, painted frontally, presents the typical Byzantine iconography of the Virgin Odigitria, who shows the Christ Child to the faithful. On her sides, the high walls of the heavenly Jerusalem extend; they symbolise the separation between the earthly space of the church and the divine space of the dome, of which the Virgin serves as a mediator and carries herself. To complete the pictorial cycle of the dome, the paintings that cover the supporting structures of the vault come into play: four eagles occupy the angular pendentives, and colourful geometric decorations enrich the four large sub-arches.
The two side walls beneath the dome are also entirely frescoed, although they are in a significantly more critical state of preservation. They depict stories, arranged in multiple registers, narrating the events of the infancy of Christ on the southern wall and His Passion on the northern one. Among the scenes that are still recognisable, on the upper right, there is the Annunciation, and further down, the Presentation of the Infant Jesus at the temple. On the left wall, the visible scenes of Jesus' Passion include His Entry into Jerusalem and the Crucifixion below.
Above these two paintings, there is a fragment of a painting executed in the early 14th century that depicts a Franciscan friar preparing to feed a bearded man with a small spoon who's wearing a curious headgear from which red hair strands are visible. Beneath that, a fragment of fresco dating back to the 15th century shows a noble patron in profile, dressed in the typical fashion of the time, with his family in the background, in an act of offering.
With the exception of these two later fragments, the overall painting campaign of the dome and walls appears unified and homogeneous in style, iconography, and technique, reflecting a strong Byzantine influence that was widespread in Lombardy during the 12th-13th century.