Exploiting this potential flaw in the defences, Caesar stationed archers and balista near the river to cover any attempt to gather water from this main source. More troublesome for Caesar however, a secondary water source flowed down from the mountain directly underneath the walls of the fort. It seemed to be almost impossible to block access to this second source. The terrain was extremely rugged and it would not have been feasible to take the ground by force. Before long, Caesar was informed of the location of the source of the spring. With this knowledge, he ordered his engineers to build a ramp of earth and rock that could support a ten-story siege tower, which he used to bombard the spring source. Concurrently, he had another group of engineers build a tunnel system that finished at the source of the same spring. The Gauls, falling for the diversion of the tower, attacked it immediately, setting it on fire and keeping it under a constant barrage of missiles to prevent the Romans from extinguishing the fire. Caesar then sent a diversionary attack up the ramp, the narrowness of the works ensuring a bloody confrontation. After a good deal of fighting on the ramp, Caesar ordered his legions in positions surrounding the city to take up a war shout, fooling the Gauls into believing a direct assault on the walls was forthcoming. The Gauls were obliged to retire from their attacks and to man the walls. Shortly thereafter, the sappers tunnelled through to the water source and finished the job of cutting the Gauls off from their water sources, forcing the Gauls to surrender their unfavourable position. Caesar accepted the Gallic surrender. However he decided to ensure that this would mark the last Gaulish rebellion by setting a severe example. He decided against executing or selling the survivors into slavery, as had been customary in contemporary battles. Instead, he had the hands of all the surviving men of military age cut off, but left them alive. He then dispersed the vanquished Gauls throughout the province for all to see that they would never again be able to take up arms against him or the Roman Republic. After dealing with the Gaulish rebels, Caesar took two of the legions and marched with a view to spend the summer in Aquitania which he had previously not visited. He briefly passed through the city of Narbo Martius in the Roman province of Gallia Narbonensis and marched through Nementocenna. Deeming Gaul sufficiently pacified, as no further rebellions arose, Caesar took the 13th Legion and marched to Italy, where he proceeded to cross the Rubicon and start the Great Roman Civil War on 17 December 50 BC.