Alesia, like Gergovia, is on a hill sloping off all round, with steep and, in places, precipitous sides. It lies between two small rivers, the Ose and the Oserain, both of which fall into the Brenne, and thence into the Seine. Into this peninsula, with the rivers on each side of him, Vercingetorix had thrown himself with eighty thousand men. Alesia as a position was impregnable except to famine. The water-supply was secure. The position was of extraordinary strength. The rivers formed natural trenches. Below the town to the east they ran parallel for three miles through an open alluvial plain before they reached Brenne. In every other direction rose rocky hills of equal height with the central plateau, originally perhaps one wide table-land, through which the water had ploughed out the valleys. To attack Vercingetorix where he had placed himself was out of the question; but to blockade him there, to capture the leader of the insurrection and his whole army, and so in one blow make an end with it, on a survey of the situation seemed not impossible. The Gauls had thought of nothing less than of being besieged. The provisions laid in could not be considerable, and so enormous a multitude could not hold out many days. At once the legions were set to work cutting trenches or building walls as the form of the ground allowed. Camps were formed at different spots, and twenty-three strong block-houses at the points which were least defensible. The lines where the circuit was completed were eleven miles long. The part most exposed was the broad level meadow which spread out to the west toward the Brenne river. Vercingetorix had looked on for a time, not understanding what was happening to him. When he did understand it, he made desperate efforts on his side to break the net before it closed about him. But he could do nothing. The Gauls could not be brought to face the Roman entrenchments. Their cavalry were cut to pieces by the German horse. The only hope was in help from without, and before the lines were entirely finished horsemen were sent out with orders to ride for their lives into every district in Gaul and raise the entire nation. The crisis had come. If the countrymen of Vercingetorix were worthy of their fathers, if the enthusiasm with which they had risen for freedom was not a mere emotion, but the expression of a real purpose, their young leader called on them to come now, every man of them, and seize Caesar in the trap into which he had betrayed himself. If, on the other hand, they were careless, if they allowed him and his eighty thousand men to perish without an effort to save them, the independence which they had ceased to deserve would be lost forever. He had food, he bade the messengers say, for thirty days; by thrifty management it might be made to last a few days longer. In thirty days he should look for relief.