The Baroque altar incorporates a lunette dating back to roughly 1350 with a Madonna and Child in the centre, St. John the Baptist on the right and a saint believed to be the martyr St. Catherine of Alexandria on the left. This fresco has been studied by many experts. Giuseppe Martinola was struck by its “vivid colours, the elegant hands and the restrained naturalism of the Child”; Carla Travi has praised “the special technique using thin and thick strokes that give the figures volume and brightness”; Piero Bianconi particularly liked that of St. Catherine, who “raises with great passion her young full face, delicately painted in pink and framed with thick blond hair”.
As for the creator of these frescoes, Michele Boskovits first attributed them to Giovanni da Milano in 1971, and since then other critics have confirmed this. A successful student of Giotto, this artist also painted the cycle of Madonna frescoes in the church of Santa Croce in Florence.