The Romans started in the first century to build fortifications along the border. One of those borders was later called Limes Germanicus. It streches from the North Sea near Katwijk to the Danube near Regensburg.
Osterburken has the remnants of such a castel (fortification)., see Extra.
Osterburken also has a Limes museum, really worth visiting. The webpage is in German only.
Many watchtowers were built between the castels. My blip shows a replica of such a watchtower. At the museum you can buy a coin (1€) needed to enter the tower.
Some days ago blipper RenFreBar visited the Saalburg, also on the Limes. Worth a visit too.