In 52 B.C. Julius Caesar, near the end of his war against Gaul, had one great enemy left in his path – Vercingetorix. The latter was a young Arvernian - charismatic, confident, and incited against Rome. Expelled from Gergovia, for being too rash, Vercingetorix raised an army on his own, and assumed the role of commander. His strategy against Caesar was simple -- use superior cavalry to harass the Romans and drive them away. Caesar, understanding his own weakness, compensated by recruiting Germans to strengthen his own cavalry units. Then, after a series of reversals, Vercingetorix was forced to retreat to the walled city of Alesia for protection. Alesia had a five mile perimeter wall six feet high with a trench in front of it. The surrounding terrain favored the defenders because it was uneven with many hills and small rivers.
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.
When this inner defense was complete, Caesar ordered the construction of a fourteen mile outer wall to protect against attack from reinforcements. The outer wall also had twin trenches on the outside.
In time, a relief force of eighty thousand Gauls arrived. Both Gallic forces attacked the Romans – the besieged army from the inside and the reinforcements from the outside. Caesar sent his cavalry against the relief force while his army fought off an attack from Vercingetorix' army. Neither Gallic army was able to penetrate the Roman fortifications. The next day Vercingetorix concentrated a new attack force against a weak spot in the inner defense. His army successfully broke through but was attacked from behind by Roman cavalry that had ridden around the outer ring to their rear. Caesar, himself, appeared with the cohorts trying to close the gap and the Romans were ultimately successful.
With their reinforcements routed, and no further hope of breaking the siege, Silesia surrendered and handed over Vercingetorix to Caesar, who had him paraded in Rome before his execution.