As the plateau was near the road connecting Châlons-sur-Saône with Langres, Cæsar judged it desirable to have at this point, which was naturally favourable for defence, a safe retreat for a numerous body of troops, more especially as the road passed through somewhat disturbed countries. The camp was to be sufficient in case of need for two legions and some auxiliaries--about twenty thousand men. Now, as the site of the Oppidum was much more extensive than was needed for a force of that strength, it was determined that the camp should be placed on the southern part of the plateau, whose level was elevated some few feet above the northern point, and which was separated from this extremity by a wide ditch.
[Illustration: FIG. 14.--THE ROMAN PERMANENT CAMP.]
Fig. 14 gives a plan of the arrangement. A ditch thirty feet wide and seven feet deep divided the plateau obliquely from W.N.W. to E.S.E. At A was placed the Prætorian gate, and at B the Prætorium. At D was the Decuman gate. The two lateral gates, F, E, fronted east and west respectively. The sunken roadways of the Gauls had to be altered and made into metalled roads; they started right and left of the bridge, C, and followed the acclivities of the plateau, rising till they branched off at O into the road from Châlons to Langres. From these two lateral military roads there was an ascent to the gates E, D, F, the _place d'armes_, H, and the two side entrances of the small advanced camp, I, on the south point of the plateau. Thus the outer circuit could be traversed without difficulty.
The ramparts of the town occupying the southern declivities of the promontory were destroyed, and the inhabitants obliged to settle on the other side of the river, either to the south-east or to the south. The head of the bridge, C, was repaired.
The gates of the camp had good _claviculæ_, each with two towers constructed of dry walling, earth, and timber work (Fig. 15).
At regular intervals along the _vallum_--which, except the front on the N.N.E., exactly followed the edge of the plateau--were erected towers, or rather watch-towers, of timber. In addition to the supply from the wells within the limits of the camp, the Romans collected the water of the springs on the northern plateau, by means of pipes made of trunks of trees bored lengthwise and joined end to end. This channel followed the roadway G, and conducted the water into six good cisterns, hollowed out in the rock and lined with cement. There was a cistern under the Prætorium, and two for each of the legions.