The unsung heroes in Caesar’s Gallic campaigns were the Roman engineers. They designed and constructed the siege lines, built the assault engines and even determined the source of the mountain springs that supplied water to the Gallic fortresses. They actually diverted the springs and by the time of the definitive battles, the Gauls were dry and thirsty. Without competent combat engineers a huge army is not effective and very vulnerable.
And, more broadly, the Roman army had the organization, logistics, training, weapons, and discipline to carry out Caesar’s extraordinary plan for double seige lines. The Gauls must have been astonished to see what the Romans were up to.
Yes, the engineering genius of the Roman’s was a large part of their success from the Punic wars on. The legions at this point, had undergone the Marian reforms, and were professional soldiers as well as engineers. They spent more time building palisades and ditches and marching than they did actually fighting. The fortifications built for this battle were as quick as the Army Corps of Engineers could do it imho. And one of the greatest ad hoc defenses ever built in world history.