The demolition of San Pietro cathedral, decided by the Napoleonic authorities in 1803, deprived Alessandria of one of its most famous and ancient buildings. A project promoted by the town council and the Fondazione Cassa di Risparmio di Alessandria, in collaboration with the Superintendency, aimed to study the ruins and restore them to public use through a series of open area excavations, which began in 2006. Despite the new administration’s decision to suspend the project, excavations were carried out in a large sector of the church: the north aisle, part of the church courtyard and the civic tower. The complex stratigraphic sequence attested the building’s centuries old history, through construction phases and architectural transformations that were largely grouped in three main periods.

The construction of the church, with a central nave and two lateral aisles began in the second half of the 12th century, when the town was founded (1168). The work continued in the 13th century giving the building the layout that it kept until its demolition. The foundations were built in a previously uninhabited area (but occupied by a cemetery), and were formed of large squared sandstone blocks with a core of cobbles and chippings from the stone working. A bell-tower aligned with the facade abutted the north perimeter wall: the upper levels of the tower (which is thought was of considerable height) were reached by a spiral staircase built into the thickness of the wall. The excavations in the north aisle revealed a portal, built into the wall, its splay decorated with small columns. Although excluded from the stratigraphic excavation, the supports sub-dividing the central nave from the lateral aisles were polystyle. To the exterior, a cobbled paving was exposed. The presence of various randomly placed postholes may indicate that the space could have been periodically occupied by timber structures for trade (market stalls?). At the front of the church, the courtyard was occupied by several “privileged” tombs, such as the T.3 (13th-14th century) which, abutted the facade and were covered by stone slabs. They would have held multiple inhumations.
More substantial transformations occurred in the 15th century when, part of the northern perimeter wall was demolished and three new chapels were built (the westernmost, that of the Madonna “dell’uscetto”, according to the sources, also had a lateral entrance); on the same side, an earlier chapel was demolished (dedicated to the Madonna della Salve) to make room for the sacristy. In the aisle, the laying of a new floor in 1591 cancelled the traces of earlier interventions; the external spaces were paved.

Between the 17th and 18th centuries, the floors were raised in order to create new tombs and funerary chambers (one belonging to the Guasco family occupied the central nave). A bell-casting pit (also attested by documents in the first half of the 17th century) and the modification of the new sacristy (1695) also date to this period. The exterior spaces seemed to maintain their commercial function in this final phase