Vallum

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.

The Roman vallus, on the contrary, presented no convenient handle, required very great force to pull it down, and even if removed left a very small opening.

While on the march, each soldier carried three or four strong wooden stakes each at least 1.5 m (5 ft) long and pointed at both ends.

It is clear that these could not have been used on their own to form the palisade of the vallum constructed around a temporary marching camp; they would have had gaps between wide enough for an attacker to pass through with ease.

One suggestion is that the "waist" facilitated them being tied together loosely in threes to form a kind of caltrop or Czech hedgehog that could be placed on the rampart (agger) of the vallum.

In the operations of a siege, when the place could not be taken by storm, and it became necessary to establish a blockade, this was done by drawing defences similar to those of a camp around the town, which was then said to be circumvallatum.

This kind of circumvallation, which the Greeks called ἀποτειχισμός and περιτειχισμός, was employed by the Peloponnesians in the siege of Plataea (Thucyd.

Their lines consisted of two walls (apparently of turf) at the distance of 16 feet, which surrounded the city in the form of a circle.

This description would almost exactly answer for the Roman mode of circumvallation, of which some of the best examples are that of Carthage in modern-day Tunisia by Scipio (Appian, Punic.