In 52 B.C. Julius Caesar, near the end of his war against Gaul, had one great enemy left – the charismatic Arvernian, Vercingetorix. 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. After a series of reversals, Vercingetorix was forced to retreat to the walled city of Alesiawith his army of 80,000.
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 had 12 legions with auxiliaries ready to bring to bear on the enemy. It was mid-summer, 52 B.C
For this post we focus on the engineering aspects of the battle, as we did with the Masada and Rhine bridge posts. Here again the tenacity of the Roman people and the skill of their engineers would provide the margin of victory.
Let’s start with The Conquest of Gaul Book 7 chapter LXIX to set the scene.
“The town itself was situated on the top of a hill, in a very lofty position, so that it did not appear likely to be taken, except by a regular siege. Two rivers, on two different sides, washed the foot of the hill. Before the town lay a plain of about three miles in length; on every other side hills at a moderate distance, and of an equal degree of height, surrounded the town. The army of the Gauls had filled all the space under the wall, comprising the part of the hill which looked to the rising sun, and had drawn in front a trench and a stone wall six feet high. The circuit of that fortification, which was commenced by the Romans, comprised eleven miles. The camp was pitched in a strong position, and twenty-three redoubts were raised in it, in which sentinels were placed by day, lest any sally should be made suddenly; and by night the same were occupied by watches and strong guards.”
Before the circumvallation could be completed, however, Vercingetorix sent a party of tribal leaders through the breech on a mission to recruit allies and bring them back as reinforcements. We move on to chapter LXXII.
“Caesar, on learning these proceedings from the deserters and captives, adopted the following system of fortification; he dug a trench twenty feet deep, with perpendicular sides, in such a manner that the base of this trench should extend so far as the edges were apart at the top. He raised all his other works at a distance of four hundred feet from that ditch; [he did] that with this intention, lest (since he necessarily embraced so extensive an area, and the whole works could not be easily surrounded by a line of soldiers) a large number of the enemy should suddenly, or by night, sally against the fortifications; or lest they should by day cast weapons against our men while occupied with the works. Having left this interval, he drew two trenches fifteen feet broad, and of the same depth; the innermost of them, being in low and level ground, he filled with water conveyed from the river. Behind these he raised a rampart and wall twelve feet high: to this he added a parapet and battlements, with large stakes cut like stags' horns, projecting from the junction of the parapet and battlements, to prevent the enemy from scaling it, and surrounded the entire work with turrets, which were eighty feet distant from one another.”
Then the Romans began to construct the countervallation.
“It was necessary, at one and the same time, to procure timber [for the rampart], lay in supplies of corn, and raise also extensive fortifications, and the available troops were in consequence of this reduced in number, since they used to advance to some distance from the camp, and sometimes the Gauls endeavored to attack our works, and to make a sally from the town by several gates and in great force. On which Caesar thought that further additions should be made to these works, in order that the fortifications might be defensible by a small number of soldiers. Having, therefore, cut down the trunks of trees or very thick branches, and having stripped their tops of the bark, and sharpened them into a point, he drew a continued trench everywhere five feet deep. These stakes being sunk into this trench, and fastened firmly at the bottom, to prevent the possibility of their being torn up, had their branches only projecting from the ground. There were five rows in connection with, and intersecting each other; and whoever entered within them were likely to impale themselves on very sharp stakes. The soldiers called these "cippi." Before these, which were arranged in oblique rows in the form of a quincunx, pits three feet deep were dug, which gradually diminished in depth to the bottom. In these pits tapering stakes, of the thickness of a man's thigh, sharpened at the top and hardened in the fire, were sunk in such a manner as to project from the ground not more than four inches; at the same time for the purpose of giving them strength and stability, they were each filled with trampled clay to the height of one foot from the bottom: the rest of the pit was covered over with osiers and twigs, to conceal the deceit. Eight rows of this kind were dug, and were three feet distant from each other. They called this a lily from its resemblance to that flower. Stakes a foot long, with iron hooks attached to them, were entirely sunk in the ground before these, and were planted in every place at small intervals; these they called spurs.
…After completing these works, having selected as level ground as he could, considering the nature of the country, and having enclosed an area of fourteen miles, he constructed, against an external enemy, fortifications of the same kind in every respect, and separate from these, so that the guards of the fortifications could not be surrounded even by immense numbers, if such a circumstance should take place owing to the departure of the enemy's cavalry; and in order that the Roman soldiers might not be compelled to go out of the camp with great risk, he orders all to provide forage and corn for thirty days.”
In late September, a relief force of eighty thousand Gauls arrived and both Gallic forces attacked the Romans – one from the inside and one from the outside. Caesar sent his cavalry against the relief force while his army fought off an attack from those trying to breakout from the city. Neither Gallic army was able to penetrate the fortifications. The next day Vercingetorix concentrated a new attack force against a weak spot in the inner fortifications. His army successfully broke through but were attacked from behind by Roman cavalry that had ridden around the outer ring to their rear. Caesar, himself, appeared with the troops trying to close the gap and the Romans were ultimately successful.
With their reinforcements routed, and no further hope to break the siege, Silesiasurrendered and handed over Vercingetorix to Caesar, who imprisoned him for six years and then paraded him through Rome before his execution.