B and F = Bridge and fort.
I recommend highly and without qualification or reservation these two short You Tube video.
Roman engineering techniques highly advanced for the time. Only the Chinese military during the same historical period was as good [if not better]?
Roman bridges, roads, forts, etc. A show of force and a scare tactic even without combat having been waged? The barbarians in awe at the ability of the Roman? Roads for instance constructed in a manner as straight as possible, nature and the lay of the land be damned!
I was aware of this bridge as built by the legions under the command of Caesar but had only thought of the structure as less than permanent. More of a pontoon-type river crossing military expedient. This is obviously not true. The same bridge could not be built in ten days using the same tools available the Romans? No! The Amish people of USA alone among moderns would have been able to replicate the feat! Bridge infrastructure embedded to protect pilings and piers from debris floating downstream. Caesar campaigning for only a short time on the far side of the Rhine and then withdrawing in good order, destroying the bridge behind him. Again that bridge primarily [?] a means to intimidate the barbarian German.
The Roman soldier among many things a construction worker. Building of such forts good training and physical development. A person during that time period TWICE as strong pound for pound as a modern human. Persons of the period used to hard physical labor very taxing. Such forts the construction of which having a lot of fore thought aspects of design conducive to a successful defense. During the siege of Alesia Caesar having to fight in two directions simultaneously, moat and wall with towers facing both inward and outward. Such construction also a clear and intimidating message to the barbarian locals that Rome was here to stay and you had better get used to it.