In the last millennium, the history of Bagnaria has been indirectly related to events taking place in Tortona or to the noble families that lived it the area at the time. It is therefore quite difficult to put the fragmented records together. The archaeological site known as Castelliere del Guardamonte, or Monte Vallassa, built on fossiliferous soil, was discovered in 1951. Some hunters dug around a fox’s earth and found fragments of earthenware. Further excavations, albeit limited in general, brought to light numerous archaeological finds. The oldest items go back to the Early Iron Age. The archaeological importance of Monte Vallassa was in a way anticipated in 1915, when a dram (unit of weight) dating from the first quarter of the 2nd century B.C., was found accidentally.