Across much of Europe the Roman Frontier followed the river Rhine and the river Danube. In order to shorten communications and gain control over certain agriculturally rich areas, the Romans created a land frontier over 500 km long with some 900 watchtowers and 120 forts between the upper reaches of the Rhine and the Danube. Construction began under the orders of Emperor Hadrian in the second century, but by the middle of the third century the frontier had been largely overrun by barbarian tribes.
More a guarded border line than a military defence system, the Limes enabled traffic to be managed, movement of people controlled, and goods traded and taxed. Increasing pressure caused by barbarian tribal movements from the east, and internal conflicts within the Empire itself led to the collapse of this part of the frontier in the late third century AD.