Quadriburgium: Roman fortification along the Rhine limes, modern Qualburg in Germany.
Excavations near the modern village Qualburg, situated on a low hill near the Rhine, have shown Roman military presence after the Batavian revolt (69-70 CE) but it is not clear what type of settlement this was, although it is likely that it had something to do with the defense of the Rhine frontier. After the mid-third century, Qualburg was fortified and served as base for an auxiliary unit (the numerus Ursariensium), but the fort was destroyed by the Franks in 275.
The Qualburg settlement was rebuilt when the emperor Probus (r.276-282) restored order along the Rhine, or perhaps a bit later. The site appears to have occupied an area of 1¼ hectare and was surrounded by a moat. Scarcity of funds suggests that the outpost was out of use during the reign of Constantine I the Great (r.306-337) but it was certainly used again in the second half of the fourth century. It was still in use in the fifth century and taken over by the Franks in the sixth century. The modern church may occupy the site of an ancient sanctuary.
The site is probably identical to the Quadriburgium (litt. "square castle") that was visited by the Roman general Julian (the future emperor) during his campaign against the Franks in 359.