Vercingetorix resolved to send away all his cavalry by night, before the Romans had completed the investment. He urges the cavalry, on their departure, to return each to his country, and recruit the men able to carry arms; he reminds them of his services, and implores them to think of his safety, and not to deliver him as a prey to the enemies, him who has done so much for the general liberty: their indifference would entail with his loss that of 80,000 picked men. On an exact calculation, he has only provisions for one month; by husbanding them carefully, he may hold out some time longer. After these recommendations, he causes his cavalry to leave in silence, at the second watch (nine o’clock). It is probable that they escaped by ascending the valleys of the Ose and the Oserain. Then he orders, on pain of death, all the corn to be brought to him. He divides among the soldiers individually the numerous cattle which had been collected by the Mandubii; but as to the grain, he reserves the power of distributing it gradually and in small quantities. All the troops encamped outside withdraw into the _oppidum_. By these dispositions he prepares to wait for the succour of Gaul, and to sustain the war. As soon as Cæsar was informed of these measures by the prisoners and deserters, he resolved to form lines of countervallation and circumvallation, and adopted the following system of fortifications: he ordered first of all to be dug, in the plain of Laumes, a fosse twenty feet wide, with vertical walls, that is, as wide at the bottom as at the level of the ground (_see Plates 25 and 28_), so as to prevent lines so extensive, and so difficult to guard with soldiers along their whole extent, from being attacked suddenly by night, and also to protect the workmen from the darts of the enemy during the day. Four hundred feet behind this fosse, he formed the countervallation. He then made two fosses of fifteen feet wide, of equal depth,[525] and filled the interior fosse--that is, the one nearest to the town--with water derived from the river Oserain. Behind these fosses he raised a rampart and a palisade (_aggerem ac vallum_), having together a height of twelve feet. Against this was placed a fence of hurdles with battlements (_loricam pinnasque_); strong forked branches were placed horizontally at the junction of the hurdle-fence and the rampart, so as to render them more difficult to scale. (_See Plate 27._) Lastly, he established towers on all this part of the countervallation, with a distance of eighty feet between them.