The Gauls, endowed with the genius of imitation, struggled by all means possible against the wonderful perseverance of the Roman soldiers. They turned away the rams with pointed heads (_falces_)[474] by means of nooses, and, when they had once caught hold of them, they dragged them up by means of machines.[475] Accustomed to work in the iron mines, and to the construction of subterranean galleries, they skilfully countermined the terrace, and also provided their walls with towers of several stories, covered with leather. Day and night they made sallies, and set fire to the works of the besiegers. As the daily increase of the terrace heightened the level of the towers, the besieged raised theirs to the same height by means of scaffoldings; they stopped the progress of the subterranean galleries, prevented them from being advanced to the walls by trying to break them open with pointed stakes hardened in the fire (_apertos cuniculos prœusta ac prœacuta materia ... morabantur_),[476] and by throwing molten pitch and blocks of stone. The Gauls constructed their walls in this manner: beams were placed horizontally on the ground, in a direction perpendicular to the line of the enclosure,[477] at intervals of two feet from each other; they were bound together on the side of the town; by cross-beams, usually of forty feet in length, firmly fixed in the ground, and the whole covered with a great quantity of earth, except on the exterior side, where the intervals were furnished with large blocks of rock, and formed a facing. After this first layer had been well fixed and rendered compact, they raised upon it a second, absolutely similar, taking care that the beams were not exactly above each other, but corresponded with the intervals filled in with stones, in which they were, as it were, enchased. The work was thus continued until the wall had attained the required height. These successive layers, in which the beams and stones alternated regularly, offered, by their very variety, an agreeable appearance to the eye. This construction had great advantages for the defence of places: the stone preserved it from fire, and the wood from the ram; held together by the cross-beams, the beams could be neither torn down nor driven in. (_See Plate 20._)