According to the Annals of Saint Bertin, in 869 Charles the Bald commissioned a rampart of wood and stone to be built around the monastery (castellum in giro ipsius monasterii ex ligno et lapide conficere coepit), in order to protect it from attacks by the Vikings. Several sections of the ditch of Saint-Denis' first fortification have been excavated, thus handily complementing the information provided by written sources.
The structure protected an area of approximately eight hectares, and was fed by the Croult, a canal more than 6 kilometres long, that brought water from the Rouillon river. A detailed study was made of the southern gate, known as the Butcher's Gate (Porte de la Boucherie). The road from Paris, six meters wide, crossed the double ditch via an earth-filled bank flanked by retaining walls. The wooden roadway was supported by posts set vertically in the bank against the freestone facing. It crossed two channels through which the water in the ditch could flow uninterruptedly beneath. The gate itself has not yet been excavated but, since it survived up to the early modern times, its exact location is known. It was constructed inside the rampart, some twenty metres behind the ditch. This access corridor leads us to think that the rampart was reinforced by other defense works, such as a protective wall associated with earthen levees. Thanks to this fortification, Saint-Denis appears to have emerged relatively unscathed from the period of the Viking invasions.