This _oppidum_ could, apparently, only be reduced by a complete investment. The Gaulish troops covered, at the foot of the wall, all the slopes of the eastern part of the mountain; they were there protected by a fosse and by a wall of unhewn stones six feet high. Cæsar established his camps in favourable positions, the infantry on the heights, the cavalry near the watercourses. These camps, and twenty-three redoubts or blockhouses,[523] formed a line of investment of 11,000 paces (sixteen kilomètres).[524] The redoubts were occupied in the day by small posts, to prevent any surprise; by night, strong detachments bivouacked in them. The works were hardly begun, when a cavalry engagement took place in the plain of Laumes. The combat was very hot on both sides. The Romans were giving way, when Cæsar sent the Germans to their assistance, and ranged the legions in order of battle in front of the camps, so that the enemy’s infantry, kept in awe, should not come to the assistance of the cavalry. That of the Romans recovered confidence on seeing that they were supported by the legions. The Gauls, obliged to fly, became embarrassed by their own numbers, and rushed to the openings left in the wall of unhewn stones, which were too narrow for the occasion. Pursued with fury by the Germans up to the fortifications, some were slain, and others, abandoning their horses, attempted to cross the fosse and climb over the wall. Cæsar then ordered the legions, who were drawn up before his retrenchments, to advance a little. This movement carried disorder into the Gaulish camp. The troops within feared a serious attack, and the cry to arms rose on all sides. Some, struck with terror, threw themselves into the _oppidum_; Vercingetorix was obliged to order the gates to be closed, for fear the camp should be abandoned. The Germans retired, after having killed a great number of the cavalry, and taken a great number of horses.