The first nucleus of the complex (called castrum) arose for the work of the Romans in the IV-V century A.D. as one of military outposts erected for defensive purposes against the Barbarians along the strip of north-western Alps. The area of the river Olona where rises peat, said Sibrium, in Roman age was in fact a place of strategic importance both for the supply of water, both for the position along a fundamental axis of communication transalpine.
The castrum was used in the subsequent centuries even by the Goths and the Byzantines and the Lombards. It was precisely during the long period of the longobard pax that the complex of peat, losing its military purpose, acquired a civil function and, later, religious, thanks to the settlement, in the VIII century, a group of Benedictine nuns who commissioned the building of the monastery and that added to the original building, the premises that accommodated the cells, the refectory and the prayer room, in addition to a portico with three arches and, in the XI century, the small church dedicated to the Virgin. During the era franca the Seprio became the seat of a countryside, thus acquiring also a function of agricultural production; in the following centuries the site became the scene of clashes between some of the most powerful families of Milan, in particular between the Tower and the Visconti in XIII century: in 1287 Ottone Visconti, to eliminate every trace of rivals, ordered the killing of all the castrum, with the exception of religious buildings (inside of which had in the meantime been incorporated also the Roman Tower).
From the documents preserved (the first written testimonies dating back to 1049) it is possible to reconstruct the history of the monastery, particularly articulated especially in the Renaissance period. Restoring order, many noble families avvicendarono to instruct as abbess a person of your own race until you come to the Pusterla, to whom we owe the definitive transfer of the nuns in Tradate, in 1482, leaving the care of the earth to Massari. Then began the so-called “agricultural period” of the complex, until, during the Napoleonic era, in 1799, with the suppression of the religious orders peat permanently lost the status of the monastery. The entire building was thus readjusted to agricultural duties: the portico was walled, the entry of the Church extended and transformed in stock for the wagons and tools and all the frescoes were covered by a new plaster.
The church was built in several stages between the VIII and the XIII century, using stones of river origin, collected from the nearby Olona and bonded between them by sand and lime. In its interior incorporates parts of a previous ecclesiastical building: Inside the church are in fact the clearly visible remains of a square bell tower present prior to construction. The outside wall of the apse, ciottoloni, is marked by four pilasters which delimit five patterns within which they were obtained single lancet windows splayed. The upper perimeter is decorated with arches in terracotta, which create an interesting chromatic game, dear to the Lombard Romanesque. Inside the church were then found some tombs and a crypt for ambulatory, referable to the VIII century, which is accessed by two staircases of stone placed on the lateral walls. The original rectangular shape was later expanded during the XII and XIII centuries through the insertion of the apse part, erected with tuff and brick. To return the original volume of the interiors, above the crypt was positioned a loft removable in wood.
The depictions in lime are, because of their state of conservation, rather fragmented and does not allow the exact identification of the subject. There are two phases identifiable frescoes: a more ancient of the IX-X century, and a subsequent, of the XI-XIII: the latter dating are referable some fragments visible on the body of the bell tower, between which it identifies the face of Joachim, accompanied by the inscription (A)KIM. The variety of weaving external masonry testifies the tormented history that the church has undergone over the centuries.