No obstacle would deter Caesar, however. He knew direct attack was impossible because of the hilltop position of the city, so he planned a siege to starve the Gauls into surrender.
Caesar ordered construction of a ten mile perimeter defense almost completely surrounding the city. Eight camps were placed in strategic positions with twenty-three lookout towers placed at equal intervals. Noting the construction and realizing he was being caught in a trap, Vercingetorix sent out his cavalry to break through and seek reinforcements. Informed of the break-out, Caesar decided he needed a more elaborate defense. His men dug a twenty-five foot wide trench, twenty-five feet deep, with vertical sides. Six hundred yards behind this trench two others were dug: both fifteen feet wide -- one filled with water. Behind the twin trenches, a twelve foot palisaded rampart was constructed. Towers were placed at one hundred and thirty yard intervals along the wall.