OK they were private but they didnt have to be.
Im not thinking of large legionary barracks but smaller stone/concrete circular towers, rather like the east franks used to govern and extend their power and territory into what became east germany. This is what rome failed to do east of the rhine.
Well, at the Limes border the Romans later on built everything from
-watchtowers (with palisade and v-shaped ditch)
http://www.miniatures.de/ancients-2-roman-watchtower.html
- small stone forts (100-200 soldiers)
- Cohort forts (stone walls, towers and double ditches
http://en.wikipedia.org/wiki/Saalburg
- Legionary barracks / towns
http://www.apx.lvr.de/english/archaeologicalpark/rec_buildings/city+wall+and+gates.htm
So they could build smaller fortifications. But how do you supply them if the native population is unfriendly for a longer time period?
The Roman army at that time is mainly an infantry army with cavalry (no stirrups) used mainly as auxiliaries.
You also don´t have anything approaching "modern" horse collars yet. Meaning that supply transport overland involves a lot more horses or mules (with an accompanying larger military escort) than in the Middle Ages. And don´t forget, zero roads.
Likewise if you want to use river transport in Germania Magna you have to tow the ships upriver against the current. Once again involving military escorts to protect the towing teams (no horse collar) and the ships.
But concerning the region "east of the Rhine".
They didn´t built only legionary barracks there.
- At least one walled town / trading place
http://en.wikipedia.org/wiki/Waldgirmes_Forum
- Cohort sized forts (also used as supply depots) in Olfen for example
- and excavations found at least one place where construction for a watchtower / signal-tower had been started (Sparrenberger Egge near todays town of Bielefeld).
After the Varus Battle (3 Legions lost plus auxiliaries) the Roman Emperors decided that it wasn´t worth the effort. To conquer, hold and pacify Germania Magna they would have needed a much larger force.
Don´t underestimate the importance of "saddles with stirrups" for the cavalry and "horse collars" for transporting supplies or towing ships. In the Middle Ages you also had a bit more of "roads" than in Germania Magna in Roman times.
Without somewhat regular supplies even a medieval castle would have to be given up sooner or later.