The basilica is the result of the superposition of no less than three structures: an early Christian cella memoriae
(of the fourth century), a small A.D. 1000 church and a monastic church built towards the end of the eleventh century. The monks who built it belonged to the reformed Benedictine order of Cluny. Cardinal Schüster had a plaque put up on the wall of the church to record that it was in this place that the philosopher Severinus Boethius had been held a prisoner, there that he had written his De Consolatione Philosophiae treatise and had been put to death in A.D. 525. The earliest part of the basilica is a small chapel known as the cella memoriae to record the place of a martyrdom or perhaps the tomb of a martyr. It was discovered during the 1994-96 excavations and, by scholars’ estimates, a fourth century date is likely. A small west-, instead of east-oriented church has also been found. An eastern orientation became the norm for all Western churches from the sixth century onwards, and it had already been so in the Eastern Church. During the eleventh century another small church with an east-oriented apse and dedicated to Saint Mary was built atop the earlier cella memoriae .
This church and the surrounding territories belonged to some vassals of the Archbishop of Milan. Towards the close of the eleventh century the church was given to the Abbey of Cluny. When they took possession, the monks built a church in the Romanesque style with three aisles and a system of supports alternating weak and strong piers.
The two salient construction characteristics of this church are that the building blocks are brickwork and that the bricks are laid out in a herring-bone pattern. This technique made not only decorative sense but also allowed the recycling of different types of bricks from earlier or collapsed buildings. The porch, in Saltrio or Vaprio stone, was carved by one or more artists whose names are unknown to us but who were active in the twelfth century. Their sculptural themes are drawn from the Gospels of the Incarnation and Nativity of Christ and depict, on the right side, the Annunciation, and, on the left, the Death of Herod the Great. The internal part of the arched lintel features a vegetal motif, luxuriant on the right side and withered on the left. The interior of the church offers an extraordinary example of frescoed decoration, clustered within the apse. The theme is the Coronation of the Virgin and, while it is the work of an artist from the Latin West, it nevertheless shows strong influences from Byzantine sources. At the centre of the semi-dome of the apse the Coronation attracts the gaze with its surrounding saints and its backdrop of a veil decorated with geometric patterns. Below the main scene, a procession of twelve prophets show more “western” artistic features and complete this pictorial project of complex theological interpretation.