C. Julius Caesar, Gallic War

Having learned these things, he sends forward scouts and centurions to choose a convenient place for the camp. And as a great many of the surrounding Belgae and other Gauls, following Caesar, marched with him; some of these, as was afterwards learned from the prisoners, having accurately observed, during those days, the army's method of marching, went by night to the Nervii, and informed them that a great number of baggage-trains passed between the several legions, and that there would be no difficulty, when the first legion had come into the camp, and the other legions were at a great distance, to attack that legion while under baggage, which being routed, and the baggage-train seized, it would come to pass that the other legions would not dare to stand their ground. It added weight also to the advice of those who reported that circumstance, that the Nervii, from early times, because they were weak in cavalry, (for not even at this time do they attend to it, but accomplish by their infantry whatever they can,) in order that they might the more easily obstruct the cavalry of their neighbors if they came upon them for the purpose of plundering, having cut young trees, and bent them, by means of their numerous branches [extending] on to the sides, and the quick-briars and thorns springing up between them, had made these hedges present a fortification like a wall, through which it was not only impossible to enter, but even to penetrate with the eye. Since [therefore] the march of our army would be obstructed by these things, the Nervii thought that the advice ought not to be neglected by them.