It is the belief of the Gauls that their country can only be defended in the fortresses, and the example of Gergovia animates them with a generous hope; but Cæsar will attempt no more imprudent assaults. 80,000 infantry shut themselves up in the walls of Alesia, and the cavalry is sent into the whole of Gaul to call to arms, and to conduct to the succour of the invested town the contingents of all the states. About forty or fifty days after the blockade of the place, 250,000 men, of whom 8,000 are cavalry, appear on the low hills which bound the plain of Laumes on the west. The besieged leap with joy. How will the Romans be able to sustain the double attack from within and from without? Cæsar has obviated all perils by the art of fortification, which he has carried to perfection. A line of countervallation against the fortress, and a line of circumvallation against the army of succour, are rendered almost impregnable by means of works adapted to the ground, and in which science has accumulated all the obstacles in use in the warfare of sieges. These two concentric lines are closely approached to each other, in order to facilitate the defence. The troops are not scattered over the great extent of the retrenchments, but distributed into twenty-three redoubts and eight camps, from which they can move, according to circumstances, on the points threatened. The redoubts are advanced posts. The camps of infantry, placed on the heights, form so many reserves. The cavalry camps are stationed on the banks of the streams. In the plain especially, where the attacks may be most dangerous, to the fosses, ramparts, and ordinary towers are added _abatis_, wolf-pits, things like caltrops, means still employed in modern fortification. Thanks to so many works, but thanks also to the imperfection of the projectiles of that time, we see a besieging army, equal in number to the army besieged, three times less in force than the army of succour, resist three simultaneous attacks, and finish by vanquishing so many enemies assembled against it. It is a thing to be remarked that Cæsar, in the decisive day of the struggle, shut up in his lines, has become, in a manner, the besieged, and, like all besieged who are victorious, it is by a sally that he triumphs. The Gauls have nearly forced his retrenchments on one point; but Labienus, by Cæsar’s order, debouches from the lines, attacks the enemy with the sword, and puts him to flight: the cavalry completes the victory.