He built the Limes Germanicus
It is a fact that he built the Limes Germanicus. Domitian's military campaigns were generally defensive in nature, as the Emperor opposed the idea of expansionist warfare. His most major military accomplishment was the creation of the Limes Germanicus, a huge network of roads, forts, and watchtowers built along the Rhine to defend the Empire.
From 83 to roughly 260 AD, the Limes Germanicus (Latin meaning Germanic frontier) was a line of frontier fortifications that delimited the ancient Roman provinces of Germania Inferior, Germania Superior, and Raetia, dividing the Roman Empire and the unsubdued Germanic tribes. The Limes often employed a natural boundary such as a river or an earth wall and ditch with a timber barrier and watchtowers at intervals. Behind the Limes, a network of linked forts was constructed.
The path of the limes shifted over time as a result of advances and retreats caused by external threats. At its peak, the Limes Germanicus extended from the Rhine's North Sea outlet to near Regensburg (Castra Regina) on the Danube. With the exception of a gap running roughly from Mogontiacum (Mainz) on the Rhine to Castra Regina, these two major rivers provided natural protection from mass invasions into imperial territory.
