Upper German-Raetian Limes

The Limes Germanicus, or 'Germanic Limes', is the name given in modern times to a line of frontier fortifications that bounded the ancient Roman provinces of Germania Inferior, Germania Superior and Raetia, dividing the Roman Empire and the unsubdued Germanic tribes from the years 83 to about 260 AD. The frontier used either a natural boundary such as a river or typically an earth bank and ditch with a wooden palisade and watchtowers at intervals, and a system of linked forts was built behind them.

The path of the limes changed over time following advances and retreats due to pressure from external threats. At its height, the Limes Germanicus stretched from the North Sea outlet of the Rhine to near Regensburg on the Danube. These two major rivers afforded natural protection from mass incursions into imperial territory, with the exception of a gap stretching roughly from Mogontiacum on the Rhine to Castra Regina.

Source: Wikipedia