This agger consisted of a terrace about a hundred paces long, ten feet high and twenty deep, with a gap in the middle twelve paces wide. From the two sides of this interval extended at right angles two galleries (vineæ), solidly constructed with trunks of trees and covered in; these galleries were about a hundred paces long. The agger was made of trunks of trees piled up, mingled with earth, with inclines for reaching the summit. This was a work of some days; and as during that time the Romans made no attack, and thought only of protecting those engaged in it from the missiles thrown from the ramparts, the besieged did not cease to ridicule them (for they were within hearing), asking them if they were intending to build a city and pass the winter there. But one morning the warriors of the Oppidum saw a wooden tower rising at the extremity of the two galleries. This tower, the woodwork of which had been prepared beforehand, was set up within the day; its summit rose more than ten feet above the top of the towers of the rampart ([Fig. 10]).
Fig. 10.
The Gauls contemplated this structure with uneasiness, although they did not comprehend its importance; so Catognatus resolved to set fire to the works during the night. With this view he had placed on the ramparts, behind the wattling, barrels filled with pitch, grease, and dry sticks; then he placed two bodies in readiness to go out by the eastern and western gates, provided with vessels full of resin, tow, and grease. These troops were to make their way along the outside of the defences, and simultaneously attack the flanks of the besiegers, while the men posted on the ramparts were to remove a portion of the wattling, and to roll the barrels, after setting them on fire, against the agger whose front was raised on the counterscarp of the ditch.
Fig. 11.
The Romans had been able to see these preparations from the summit of the tower; moreover, they stationed a strong guard on the flanks of the plateau every evening. The legate at nightfall had these points protected by stakes, and had a quantity of stimuli ([Fig. 11]) driven into the ground outside. About the third hour of the night the besieged issued noiselessly from the two gates and came within half a bowshot of the Roman posts. At a signal given from the interior, the two bodies rushed at once on the besiegers' flanks. But even before they could reach the palisades, many of them, wounded by the stimuli, fell uttering cries of pain. Those who reached the besiegers' posts, thinned by the darts showered upon them, and hesitating on seeing so many of their party fall, were more inclined to fly than to continue the attack when they saw themselves in their turn taken in flank and rear by the besiegers. The defenders on the ramparts, whom the darkness of the night prevented from seeing clearly what was taking place, and not knowing whether the confusion arose from the flight of the Romans or of their own men, dared not discharge stones and arrows.
Meanwhile the lighted barrels were being rolled in front of the galleries, which had already begun to take fire. By the glare of the flames they could see the Roman soldiers mount on the terrace carrying baskets full of wet earth, which they threw on the barrels; and the defenders killed or wounded many of them. At this moment some of the fugitives who had taken part in the two sorties, re-entered the camp calling out that they were pursued by the Romans.