The Limes Germanicus was 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 for more than 200 years. At its height, the Limes stretched from the North Sea outlet of the Rhine to near Regensburg (Castra Regina) on the Danube. Those two major rivers afforded natural protection from Barbar incursions into Imperial territory.
The total length was 568 km. It included at least 60 forts and 900 watchtowers. The potentially weakest, hence most heavily guarded, part of the Limes was a small gap between the westward bend of the Rhine at modern-day Mainz and the main flow of the Danube at Regensburg. This 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.
Roman border defences in modern day Germany have become much better known through systematic excavations and reconstructions financed by Germany and through other research connected to them. In 2005, the remnants of the Upper Germanic & Rhaetian Limes were inscribed on the List of UNESCO World Heritage Sites as Frontiers of the Roman Empire.