The greatest monument of Roman power in Britain and that which has yielded the most fruitful results to archæology is the Roman Wall between the two estuaries of Tyne and Solway. Almost all that we know of Roman life in Britain during the second century centres round this one great work. Towards the end of the first century a change took place in the organisation of the defence of the empire on the frontiers. Hitherto the republic, and after it the empire, had been satisfied to keep a strong body of troops in all the imperfectly conquered provinces, and to plant well-garrisoned castles near the river or the range of mountains on the other side of which were the barbarians of Europe or Africa, or the hostile monarchies of Asia. Soon after the death of Nero a different system was adopted, involving the formation of a definitely marked boundary which when not protected by very strong natural barriers was guarded by an actual wall of stone or earth upon which the garrisoned fortresses were strung, like beads on a chain. Not only in Britain are traces of these limiting walls to be found, but also in Germany, between the Lower Rhine and the Danube, and in the Dobrudscha on the western shore of the Black Sea: and there is reason to believe that a similar wall of defence shut out the barbarians of Mount Aures who threatened the provincials of Roman Africa. “The real authors of the frontier system were the Flavian and Antonine Emperors, and the period extending from the accession of Vespasian to the death of Marcus Aurelius, or, roughly, from 70 A.D. to 180 A.D., witnessed its complete organisation. The interest of these emperors in the matter was no doubt quickened by the growing anxiety, an anxiety unknown to the Augustan age, but perceptible in Tacitus, as to the increasing pressure from without upon the empire.... It is well for students of the British frontier to remember that the emperor with whose name the organisation of the imperial frontier system is most closely connected is Hadrian.”[15]