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Vallum is either the whole or a portion of the fortifications of a Roman camp. The vallum usually comprised an earthen or turf rampart (Agger) with a wooden palisade on top, with a deep outer ditch (fossa). The name is derived from vallus (a stake), and properly means the palisade which ran along the outer edge of the top of the agger, but is usually used to refer to the whole fortification. Wikipedia