The fourth infantry camp was established on the lower slopes of Mont Réa, at _D_. It is the one occupied by the two legions of Reginus and Rebilus, and which Vercasivellaunus attacked with 60,000 men. Indeed, it will be observed that the spur situated to the north of Mont Auxois, between the Rabutin and the Brenne, is much farther from Alesia than the other mountains which surround it, and Mont Réa, which is the nearest part of it, is still more than 2,000 mètres distant from it. Hence it follows that Cæsar could not have included Mont Réa in his lines without giving them an excessive development. Consequently, he was obliged to establish one of his camps on the southern slope of that hill. This camp was on the point of being forced, and an obstinate battle was fought there. The excavations have led to the discovery in the fosses of a multitude of interesting objects, and, among them, more than 600 Roman and Gaulish coins. (See the list in _Appendix C_.)[537] The extremity of the upper fosse, represented by dots on _Plates 25_ and _28_, has not been discovered, because earthfalls have taken place on that part of Mont Réa, which would have obliged the excavators to dig too deep to arrive at the bottom of the fosse. The strength of the retrenchments of the infantry camps was very variable, as may be seen by inspecting the various profiles of the fosses. (_See Plate 28._) For each camp, they have larger dimensions on the side which is not defended by the escarpments, as may easily be conceived. There were four cavalry camps, _G H I K_, placed near the different streams: three in the plain of Laumes, and one in the valley of the Rabutin. The fosses of these camps took greatly varied shapes. (_See Plate 28._) In general, their dimensions were decidedly less than those of the fosses of the infantry camps. Camp _G_, however, had rather deep fosses; no doubt because it was farthest from the lines. The fosse which enclosed camp _I_ towards the side of the Brenne has disappeared by the inundations of the river. REDOUBTS, OR CASTELLA.--Of the twenty-three redoubts or blockhouses (_castella_), five only have been discovered; they were the most considerable; they are represented on _Plate 25_, by the numbers 10, 11, 15, 18, 22. The others, built of wood, and forming blockhouses, would not have left any trace; they are marked by circles on the most convenient places.