[3] Casteggio borders the following municipalities: Borgo Priolo, Calvignano, Casatisma, Corvino San Quirico, Montebello della Battaglia, Oliva Gessi, Robecco Pavese, Verretto.

Clastidium was a settlement of the Ligures, belonging to the tribe called Marici by the Romans (and wrongly identified as Celts by Polybius).

The location was the site of a major defeat of the Marici by the legions of Marcus Claudius Marcellus, which was celebrated in a tragedy by the Latin poet Naevius.

Never to recover its former splendour, it was annexed to the colonia of Piacenza, and remained under that city’s control after the fall of the Western Roman Empire.

In 1441 it was acquired by Cesare Martinengo, and was later ruled by the Simonetta, Sforza, Bentivoglio and Del Carretto dynasties.