As an ambitious general, Caesar wanted to cross the Rhine River for the purpose of attacking Germany [Gaul]. His soldiers transformed the surrounding trees into an expanding bridge that sustain the weight of 40,000 men. The four-foot piles were connected with wooden beams of wood and then other timber laid across perpendicular to the piles. The soldiers finished the job with tightly wrapped bundles of sticks. It wasn't the actual bridge that caused the excitment but rather the fact that it was build in ten days.
After Caesar and his army marched across the bridge, the German army was so impressed with the speed of the bridge building that they fled from Caesar's army. He scoured the territory for eighteen days, recrossed the Rhine River on the bridge, dismantled it and went home.
A great book for the history of this bridge and the rest of the rise and fall of the Roman Empire is A History of the Roman Peoply by Allen Ward.
Hope this helps. dorry