de bello gallico, 5.42

Rebuffed from this hope, the Nervii surround the winter quarters with a rampart of eleven feet and a ditch of thirteen feet. These also they had learned from our men in the experience of former years, and they were instructed by these whom they had as captive secretly from the army: but, as there was no supply of iron tools which were suitable for this use, they were seen to cut the turf with swords and to empty out the earth with their hands and cloaks. From which circumstance indeed, the multitude could be inferred; for in less than three hours they completed a fortification of fifteen miles in circumference: and, during the rest of the days, they began to prepare and make towers to the height of the rampart, grappling irons, and movable sheds, which the same prisoners had taught them.