At first the Romans secured natural boundaries. The built forts, fortlets and most probably watchtowers along the rivers the defined the edges of the Roman Empire. Those river frontiers were called ripa and well-known examples are the frontiers along the Rhine and the Danube. But still Rome tried to control territories beyond those frontiers. Outpost in the barbaricum are very common and by the end of the first century AD the Romans not only focused on natural boundaries, they created now artificial frontiers lines. They were now able to incorporate i.e. fertile strips of land into the empire.
Usually the artificial frontiers consisted of several elements. Forts, fortlets and watchtowers had to supervise the chosen frontier line. Later a linear barrier had to prevent uncontrolled movement. The development of the Roman frontiers culminated in the second century AD. In AD 122 the emperor Hadrian ordered the building of a frontier line in Britain from sea to sea. Hadrian's Wall was built of stone from the Solway to the Tyne, about 3 m wide and more than 4 m high. It was the strongest frontier the Romans ever built.
Author: Erik Dobat