Cæsar, having arrived on the banks of the Allier, towards Moulins (_see Plate 19_), followed its downward course, on the right bank. Vercingetorix, on his part, marched along the opposite bank. The two armies were within sight of each other, the camps nearly opposite, and the Gaulish scouts, who watched the left bank, prevented the Romans from establishing a bridge. The position of the latter was difficult, for the Allier, which is fordable in the autumn only, might delay their passage a long time.[482] In order to surmount this obstacle, Cæsar had recourse to a stratagem: he fixed his camp in a place covered with wood, opposite the remains of one of the bridges which Vercingetorix had caused to be destroyed (probably at Varennes). There he remained hidden the following day with two legions, and made the rest of the troops, as well as the baggage, proceed in the usual order. But, that they might present to the enemy the appearance of six legions, he had divided into six corps the forty cohorts or four legions which he sent forward.[483] They received the order to march as long as possible, in order to attract Vercingetorix, and, at the time Cæsar presumed that they had arrived at their camp, he caused the bridge to be rebuilt on the old piles, the lower part of which remained untouched. The work being soon completed, the legions which remained with him passed the river, and, after having chosen a favourable position, he recalled the mass of his army, which rejoined him during the night.[484] When Vercingetorix was informed of this manœuvre, fearing lest he should be compelled to fight against his will, he took the start and marched in great haste to occupy the _oppidum_ of the Arverni. From the place he occupied, which we believe to have been Varennes,[485] Cæsar reached Gergovia in five days; on the very day of his arrival, after a slight skirmish of cavalry, he reconnoitred the position of the town. As it was built on a very high mountain of difficult access, he considered it impossible to take it by assault; he therefore resolved to blockade it, and not to begin investing it until he had assured his provisions. (_See Plate 21._) [Sidenote: Blockade of Gergovia.] VI. The _oppidum_ of the Arverni was situated at a distance of six kilomètres to the south of Clermont-Ferrand, on the mountain which has preserved the name of Gergovia. Its summit, elevated about 740 mètres above the level of the sea, and 380 above the plain, forms a plateau of 1,500 mètres in length by more than 500 mètres in breadth. The northern and eastern slopes present such abrupt declivities that they defy the escalade. The southern slope presents a very different character: it may be compared to an immense staircase, the steps of which would be vast terraces with very little inclination, and a breadth which in some places extends to as much as 150 mètres.