Obergermanisch-Raetischer Limes / Limes de Germanie Supérieure et de Rhétie / Upper German-Raetian Limes

The Limes Germanicus (Latin for Germanic frontier) was a line of frontier (limes) 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. At its height, the limes stretched from the North Sea outlet of the Rhine to near Regensburg on the Danube. Those two major rivers afforded natural protection from mass incursions into imperial territory, with the exception of a gap stretching roughly from Mogontiacum (Mainz) on the Rhine to Castra Regina (Regensburg) on the Danube.

The Limes Germanicus was divided into:

• The Lower Germanic Limes, which extended from the North Sea at Katwijk in the Netherlands along the then main Lower Rhine branches (modern Oude Rijn, Leidse Rijn, Kromme Rijn, Nederrijn
• The Upper Germanic Limes started from the Rhine at Rheinbrohl (Neuwied (district)) across the Taunus mountains to the river Main (East of Hanau), then along the Main to Miltenberg, and from Osterburken (Neckar-Odenwald-Kreis) south to Lorch (Ostalbkreis) in a nearly perfect straight line of more than 70 km;
• The proper Rhaetian Limes extended east from Lorch to Eining (close to Kelheim) on the Danube.

The total length was 568 km (341 miles). It included at least 60 forts and 900 watchtowers. The potentially weakest, hence most heavily guarded part of the Limes was the gap between the westward bend of the Rhine at modern-day Mainz and the main flow of the Danube at modern-day Regensburg.

This 300-km wide land corridor between the two great rivers permitted movement of large groups of people without the need for water transport, hence the heavy concentration of forts and towers there, arranged in depth and in multiple layers along waterways, fords, roads, and hilltops.

Source: en.wikipedia.org