The Roman legions had a special class of soldiers, the so-called immunes. These were veteran soldiers who no longer (perhaps because of advanced age or injuries) were required to perform the duties regular legionaries, and were instead in charge of building the temporary castra when the legion was on the march.

Digging trenches and creating earthen ramparts was a task in which all legionaries were expected to participate (most temporary Roman forts consisted only a wooden stockade, a ditch and raised earth) but the immunes would plot out the layout of the camp, pitched the officers tents and would be in charge of the logistical process of procuring the stakes for the palissade and placing them correctly.

The campaign castrum was not a complicated structure to construct (they were mainly meant to prevent night attacks and to control an area until reinforcements arrived) so the task of the immunes was mainly to delegate and supervise. The most basic equipment needed was carried by the legionaries themselves; each legionary soldier would carry about two (relatively small) stakes needed for a basic palisade and a shovel. More advanced tools (axes, saws, etc.) would have been carried by the bagage train and under the supervision of the immunes who would have also been their principal users. They would have not been a large group within the legion as a whole; most likely they numbered about 60-80 men within a single legion.

During the Gallic Wars, Caesar indeed build large circumvallation works at Alesia. It is also true that these were huge structures: the walls he build to surround the town were 16 km long, those that he built to protect his troops against outside attacks were even larger, at 20 km. It is important to note however, that these were relatively simple structures. They consisted of ditches, earthen ramparts and palissades made from trees. For the Roman Legions at Alesia (at least 30.000 men) this would have been a problem of scale, not so much of technology.

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