Piers Major had advanced promptly upon receiving the message brought by his son. The chances of a frontal attack had already been discussed between him and Constans, and the latter had devised a formation which, in theory at least, should make such an undertaking feasible. In its basic idea it was the Roman _testudo_, described by Julius Caesar in the Gallic Commentaries. The phalanx of marching men were protected from arrows, darts, and ordinary missiles by a continuous covering formed of their ox-hide shields, the latter being held horizontally above the head and interlocked. The overlapping shields bore a fanciful resemblance to the scaly carapace of a tortoise--hence its name; and, so long as the essential principle of unity of action was maintained, it might be reckoned an effective engine of warfare. As the _testudo_ moved down the Palace Road and towards the wooden barrier of the north gate, it was to be observed that the front-rank men and the file-closers carried their shields in the ordinary fashion, in order to ward off horizontally flying missiles. Once under the shelter of the walls, the leaders would immediately discard their now useless bucklers and begin to ply their axes, protected from overhead assaults by the overlapping shields of their comrades. The formation advanced steadily; there was a suggestion of terrific irresistibility in the very slowness of its progress; to the eager fancy it might have been the veritable recreation of some prehistoric monster, the illusion being heightened by the torchlight that flickered uncertainly over the rounded bellies of the shields of greenish leather and was reflected redly from their copper bosses.