C. Julius Caesar, Gallic War

Meanwhile, with the legion which he had with him and the soldiers which had assembled from the Province, he carries along for nineteen [Roman, not quite eighteen English] miles a wall, to the height of sixteen feet, and a trench, from the Lake of Geneva, which flows into the river Rhone, to Mount Jura, which separates the territories of the Sequani from those of the Helvetii. When that work was finished, he distributes garrisons, and closely fortifies redoubts, in order that he may the more easily intercept them, if they should attempt to cross over against his will. When the day which he had appointed with the embassadors came, and they returned to him; he says, that he can not, consistently with the custom and precedent of the Roman people, grant any one a passage through the Province; and he gives them to understand, that, if they should attempt to use violence he would oppose them. The Helvetii, disappointed in this hope, tried if they could force a passage (some by means of a bridge of boats and numerous rafts constructed for the purpose; others, by the fords of the Rhone, where the depth of the river was least, sometimes by day, but more frequently by night), but being kept at bay by the strength of our works, and by the concourse of the soldiers, and by the missiles, they desisted from this attempt.