The Danubian Limes (German: Donaulimes), or Danube Limes, refers to the Roman military frontier or limes which lies along the River Danube in the present-day German state of Bavaria, in Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria and Romania.
The Danube was not always or everywhere used by the Romans as the military frontier which was moved north or south in some locations according to military conquests, but it was maintained in many places as a fairly permanent defensive structure for long periods. The border was reinforced with numerous watchtowers, legion camps (castra) and forts (castella). Due to the boggy and dendritic nature of the Danube's river banks no border ramparts were built, unlike the Neckar-Odenwald Limes in Germany. The camps were built in the mid-1st century. Later, under Trajan, the camps, which had originally only been surrounded by earthen embankments, were enclosed by stone walls.