With the rebuilding of the adjacent monastery, the Abbey increased its possessions and influence over the territory that corresponded to it until 1303. In 1477 Sixtus IV granted the abbey in commendam and in 1517 it became the property of Francesco Cybo, passing into the hands of the municipality of Ferentillo after the unification of Italy. In the fifteenth century the cloister was added and in the early years of the sixteenth century the current façade was built, which is very sober with a Renaissance portal surmounted by a rose window.
The church, in a Latin cross, has a nave that narrows towards the transept, which is slightly raised and ends with three apses. The apses and the transept date back to the original eighth century layout. The marble slab altar originates from the ancient presbytery perimeter. Preserved inside the church are some Longobard fragments and five Roman sarcophagi with pagan subjects, except one Asian type which, according to tradition, housed the body of Duke Foroaldo II.
At the top of the side walls and on the triumphal arch there is an important cycle of frescoes from the twelfth century, with Stories from the Old and New Testaments that, due to the number of scenes and state of preservation, is considered among the most important example of Romanesque painting in Italy, representing one of the first reactions to the "hieratic" style of Byzantine painting. The remaining walls and the three apses are decorated with frescoes from the fifteenth and sixteenth centuries.
The high altar, located at the centre of the chancel and made with marble fragments dating from the eighth century, has a frontal bearing the inscription Ursus Magister, the first example of a sculpture signed by the author. The bell tower, with a quadrangular layout, rises to five floors divided by brick cornices. The walls have numerous stone fragments from Roman times.
A portal from the second half of the eleventh century leads into the rectangular cloister lined with a double loggia, the lower arches of which date from the twelfth century, while the upper ones are later. In the courtyard there is a circular marble altar, probably of Greek origin, with bas-reliefs of satyrs and maenads.
The remaining buildings of the abbey complex, which also probably date back to the twelfth or thirteenth century, are arranged around the cloister