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. It is likely that these would be augmented with whatever was to hand, such as tree branches or thorn bushes. Usage 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. Such a circumvallation, besides cutting off all communication between the town and the surrounding country, formed a defence against the sallies of the besieged. There was often a double line of fortifications, the inner against the town, and the outer against a force that might attempt to raise the siege. In this case the army was encamped between the two lines of works. Construction This kind of circumvallation, which the Greeks called ἀποτειχισμός and περιτειχισμός, was employed by the Peloponnesians in the siege of Plataea (Thucyd. ii.78, iii.20‑23). 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. Between the walls were the huts of the besiegers. The walls had battlements (ἐπάλξεις), and at every ten battlements was a tower, filling up by its depth the whole space between the walls. There was a passage for the besiegers through the middle of each tower. On the outside of each wall was a ditch (τάφρος). 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. 119, &c.), that of Numantia in modern-day Spain by Scipio (Appian, Hispan. 90), and that of Alesia in modern-day France by Caesar (Bell. Gall. vii.72, 73). The towers in such lines were similar to those used in attacking fortified places, but not so high, and of course not moveable. (Lipsius, de Milit. Rom. v.5, in Oper. iii. pp156, 157; Poliorc. ii.1, in Oper. iii.283). The vallum-building technique was later taken by neighbouring people, such as the Byzantines and the Goths. Examples Examples of valla include: "The Vallum", a component of Hadrian's Wall (England, Roman) Trajan's Wall (Romania, Byzantine Age) Athanaric's Wall (Romania, Moldova, 2-4th century, probably made by the