It is astonishing that these earth levees that protected the oppida have withstood the test of time so well. This is probably due to how they were built, since these are not just heaps of dirt. Caesar gave a very good description of how this type of "murus gallicus" was built, which was confirmed by archaeological excavation. Earth and loose stones were packed into frames built from large vertical and (most importantly) horizontal beams. These beams were held together by huge nails or sheets of metal. These constructions were remarkably cohesive, and held up well against Roman war machinery — especially since they were sometimes reinforced by external facing, like at La Chaussée-Tirancourt (Somme). In the seventeenth and early eighteenth century, archaeologists were very much interested in these fortified ensembles. These oppida were striking because of their size, which artists did not hesitate to exaggerate in drawings and paintings!
Without a doubt, many of these oppida predate the conquest of Gaul. At times, they were built over Bronze Age entrenchments, which is the case, for example, at Vieux-Moulin (Oise). Recent excavations have confirmed pre-conquest construction, like at La Cheppe (Marne). But it is taken for granted that the Romans re-occupied certain oppida. Recent research appears to indicate that they sometimes reinforced them and even perhaps, in rare cases, built them. Thus, at La Chaussée-Tirancourt (Somme), it seems that the Romans adopted the sophisticated and efficient murus gallicus construction techniques.
Large earth levee of the "Camp of Caesar" at Liercourt-Erondelle (Somme).
Overall view of this protected spur with its naturally steep walls. The arrows indicate the large levee of earth. Liercourt-Erondelle (Somme).
There are still questions concerning the chronology of the oppida. Some played a role during the Gallic Wars and even later. For La Chaussée-Tirancourt (Somme), the re-occupation of the oppidum continued until about 30–20 BCE. Could it be that, in reality, that this was entirely the work of the Romans, who built a murus gallicus themselves? (Fichtl, 1994). Clearly, Caesar had been impressed by the ingenuity and efficiency of this type of fortification.