Peace and prosperity of the empire depended on the defense of the frontiers which are guarded by forts, watchtowers, and ramparts.
In the middle decades of the 3rd century AD, Rome had no mobile army held in reserve, and military units were concentrated in camp and forts along the fields.
It was the frontiers which gave the province security and allowed economies to flourish and provide taxes for the imperial army. At first chains of forts and watchtowers were linked by a military road later crucial sections of the frontier were fortified by building a wall. Most famous was The Hadrian Wall, 70 mile (112km) stone wall from north of the River Tyne to the Solway Estoarery and extended down the Cumbian coast.
On the European mainland, there was no need for continuing the barrier. The frontier ran along the Rhine and the Danube and those were sufficient obstacles. Forts, camps, and watchtowers were built along their banks. New frontier defenses continued to be built during the late Roman period. This included a series of boundary earthworks, the Devil's Dykes and Bradza lui Novac du Nord, built by Constantine (306-337 AD) to protect the tributary peoples beyond the Danube from the Goths.