Thanks for posting. How in the world Caesar’s engineers built two bridges so quickly from nearby materials is still unknown.
I was wondering how they would have driven the pilings into the river bottom. I think I saw how on a documentary once, but my memory fails me. I’m guessing they erected a tripod centered above the piling, had a team of horses pull a rope attached to a boulder and up through the tripod apex, and then released the boulder to fall on the top of the piling—again and again.
Bridge building
Trajan’s Bridge across the Danube, the longest bridge for over a millennium
Further information: Roman bridge
The engineers also built bridges from both timber and stone depending on required permanence, time available etc. Some Roman stone bridges survive to this day. Stone bridges were made possible by the innovative use of the keystone to allow an arch construction. One of the most notable examples of military bridge-building in the Roman Empire was Julius Caesar’s Bridge over the Rhine River. This bridge was completed in only ten days and is conservatively estimated to have been more than 100 m (300 feet) long.[1][2] The construction was deliberately over-engineered for Caesar’s stated purpose of impressing the Germanic tribes,[3] who had little experience of engineering, and to emphasise that Rome could travel wherever she wished. Caesar was able to cross over the completed bridge and explore the area uncontested, before crossing back over and dismantling the bridge. Caesar relates in his War in Gaul that he “sent messengers to the Sugambri to demand the surrender of those who had made war on me and on Gaul, they replied that the Rhine was the limit of Roman power”. The bridge was intended to show otherwise.
https://en.wikipedia.org/wiki/Roman_military_engineering
They just knew how to do it!