Caesar quickly grasped the changed situation and followed, immediately beginning on his arrival that inexorable enclosure of his enemy that would isolate Vercingetorix's army from its remaining allies.
by Julius Caesar:
The siege works that we were beginning to build formed a circuit of 18 kilometers. Camps were constructed at strategic points along it, and we built 23 redoubts there as well. Pickets were stationed in these during the daytime to guard against any sudden breakout from the oppidum; at night they were occupied by strong garrisons with sentries on watch.
When I was informed of this by fugitives and prisoners, I began building siege works of the following kind. I had a trench dug 20 feet wide, with perpendicular sides so that it was as broad at the bottom as it was at the top. Then I moved all the other siege works back 600 meters from this trench. I did this to counter certain difficulties: the area to be enclosed was very wide and it would not be easy to man the whole circuit; the enemy might suddenly swoop down en masse on our fortifications at night, or they could possibly, during the daytime, hurl their weapons at our men while they were busy and occupied with the work.
So, at this distance of 600 meters, I had two trenches dug, of equal depth and each 5 meters wide. The inner one ran across the plain and the low ground, so I filled it with water diverted from the river. Behind these trenches, I erected a rampart and palisade 4 meters high. To this I added a breastwork with battlements, with large forked branches projecting at the point where the breastwork joined the rampart, to stop the enemy if they tried to climb up. Finally, I had turrets erected at intervals of about 27 meters along the entire circuit of our fortifications.
A whole lot of digging and building on the part of his Legions, but they were Combat Engineers as well as Heavy Infantry and it did cut down on Roman casualties.