The original octagonal floorplan of the church shows that the building was intended as the chapel of the Theatine monastery. Richly adorned with eighteenth-century work in polychrome marble, the interior is enlivened by the fascinating spatial distribution of the pilasters, which run up beyond the trabeation to meet at the top of the ceiling.
Subsequent additions to the late-sixteenth century interior decoration may have altered the original simplicity, but they created a church which is a splendid example of Baroque art: for example, at the cupola, whose vault is divided into sixteen equal segments decorated with a marvellous fresco of Paradise, the work of the French artist Guido Luigi Vernansal (completed in 1724).
Particular mention should be made of the Holy Sepulchre Chapel, which was built as part of the original sixteenth-century church; this bears witness to the profound contemporary devotion to the Crucified Christ and the numerous pilgrimages that were typical of the period.