In Beauvais, the existence of just two urban fortifications is generally assumed, namely the antique castrum, built between the last quarter of the 3rd and the beginning of the 4th century, and the large enclosure that appears on the oldest iconographic representations of the town dating from the 16th to 18th centuries, the construction of which is generally dated to the reign of Philip Augustus without any real proof. The article proposed here establishes the existence of an extension to the antique castrum, probably around the 9th century, and also suggests a primitive state of the urban wall built during the 12th century that is quite distinct from that shown in the earliest representations of the town. Several clues point to an extension of the original medieval enclosure during the Hundred Years’ War. None of these developments is clearly attested in written sources. The study is based on a variety of sources, including archaeological findings (which have often been reinterpreted), relatively rich archival holdings and a morphological analysis of the ancient urban parcel system.