The external can be divided into three levels: apses and corner chapels, roof and presbytery and dome. Particular attention has been paid to the position of the windows, studied in function of the internal paintings with the light that gradually spreads throughout the church. The intermediate spans are slightly shaded, the apse receives a diffused and abundant illumination, but it is the dome that is hit by the light more intensely. The arms that form the Greek cross plan are placed on the cardinal axes and closed by four large symmetrical apses, and guard four quadrangular chapels always intended for worship. The interior is decorated with frescoes of the Parma school of the seventeenth century. The Parmigianino, which was initially entrusted with the entire pictorial work, realizes only the frescoes of the eastern underworld. The work was in fact continued by Michelangelo Anselmi and Bernardino Gatti. No less interesting are the cabinet-making works for the sacred furnishings of the noble sacristy and the stalls of the choir of the knights. It is due to the will of Maria Luigia of Austria that, in 1823, a crypt was built to preserve the tombs of the dukes and princes of the houses Farnese and Bourbon Parma: in memory of the duchess was placed, at the entrance of the church, the group with the Pietà of Tommaso Bandini.