The cloister and the abbey buildings were also constructed at that time, which were built on the right-hand side of the church.
In 1957 a particularly complex and thorough restoration returned the interior to its original, more austere form, "freeing" the structure from the elaborate eighteenth-century decoration.
The church has a very narrow nave and side aisles without a transept and ending with a triconch apse.
There is a raised presbytery with the staircase restored during the renovation works in the 20th century.
The central nave is covered with a lowered barrel vaulted ceiling, whilst the side aisles have a cross vaulted ceilings. The three spaces are separated by two rows of traditionally Longobard circular pillars.
A triumphal arch which divides the nave from the presbytery has a mullioned window at its centre.
There is currently a gabled façade, a transformation brought about by adding ashlar masonry in the 16th century, although originally, as you can still see, it had a four-sided pitched roof, corresponding to the different heights of the naves.
It frames an elegant three-mullioned window with salvaged columns and capitals and a door with simple insets.
The three apses are decoratively clad externally, separated by thin pillars resting on a high base and culminating in small hanging arches.
The crypt is subdivided into three apsidal naves, with cross vault ceilings and the central nave which is in turn divided into three parts by six columns.
The crypt is characterised by capitals depicting stylized animals and plant motifs, which can be dated to the end of the 11th century.
An ancient sarcophagus containing the remains of the martyr San Felice is located behind the altar.
The portico in the rectangular cloister is supported by solid square brick pillars; above each pillar are frescoes of saints and the blessed. The frescoes on the walls depict stories from the life of San Felice.
At the centre of the cloister is a cistern which collects rainwater.
Since 1815 the abbey has been headquarters of the Missionaries of the Precious Blood, founded by San Gaspare del Bufalo.