I don't have specific information on the Rhine, but I hope this helps.

During peace time, the Roman legionnaries were used for many different duties we now think of more civilian in nature than military. For example, legionnaries were used as administrators, engineers, construction workers, police and manufacturers. Roman legionnaries were also employed as miners; General and legate of Germania superior, Curtius Rufus, got a triumph for using his troops as miners in a silver mine, and the Legio II Augusta supervised the lead mines of Mendip Hills in western Britannia.

Roman fortifications rarely were fortifications in the sense that the Romans wanted to fight inside them. In most cases, Romans sought to defeat their adversaries in an open field instead of defending a camp. Even the best known border wall, Hadrian's wall, was most likely designed to delay a possible invasion long enough that the Romans would be able to muster an adequate army to beat the enemy in an open field battle. However, the Romans would have known of any great tribal army, as the Romans also kept tight diplomatic connections to neighbouring tribes. Border walls also hampered raiding of Roman lands. Even as it was possible to climb the wall and enter Roman territory, it wasn't possible to do so with a horse, and without a horse, the amount of loot one can carry is fairly limited. Romans had a large network of watch towers and outposts at their borders and near important roads. The watch towers would have been a sign of Roman power at the borders, a sign that the Imperial Army is watching. Some of them were also a part of a network of signal towers, which would have sent messages with signal fires or by optical telegraphy.

My primary source was Adrian Goldsworthy's The Complete Roman Army.