|From Wikipedia: Milecastle 39 on Hadrian's Wall
Emperor Augustus had set the natural boundaries of the Roman Empire as being the Rhine, the Danube and the Euphrates. This policy had been abandoned by Trajan in his conquests of Dacia, Armenia, Mesopotamia, and Assyria.
On taking possession of the imperial power Hadrian at once resumed the policy of the early emperors, and devoted his attention to maintaining peace throughout the world, for the nations which Trajan had conquered began to revolt; the Moors, moreover, began to make attacks, and the Sarmatians to wage war, the Britons could not be kept under Roman sway, Egypt was thrown into disorder by riots, and finally Libya and Palestine showed the spirit of rebellion. Whereupon he relinquished all conquests east of the Euphrates and the Tigris, following, as he used to say, the example of Cato, who urged that the Macedonians, because they could not be held as subjects, should be declared free and independent.
After stabilising his imperial status in Rome, Hadrian travelled to the provinces of Gaul, and came to the relief of all the communities with various acts of generosity; and from there he went over into Germany to examine the state of the military.
Having reformed the army quite in the manner of a monarch, he set out for Britain, and there he corrected many abuses and was the first to construct a wall, eighty miles in length, which was to separate the barbarians from the Romans.
This fortification extended from Wallsend at the mouth of the Tyne to Bowness on the Firth of Solway, a distance of 73.5 English miles. Its remains show that it consisted of two lines of embankment with a moat between them, and a stone wall running parallel on the north. In the space between the embankment and the wall were small strongholds about a mile apart with an occasional larger stronghold, all connected by a military road.
After arranging matter in Britain Hadrian crossed over to Gaul, for he was rendered anxious by the news of a riot in Alexandria, which arose on account of a living manifestation of the bull god Apis. After this he travelled to Spain where he restored a temple to Augustus and extracted a levy from the natives and the Italian settlers who were previously exempt.
During this period and on many other occasions also, in many regions where the barbarians are held back not by rivers but by artificial barriers, Hadrian shut them off by means of high stakes planted deep in the ground and fastened together in the manner of a palisade.
Just such a palisade has been found on the German frontier where the rivers Main and Neckar do not constitute a natural boundary. He appointed a king for the Germans, suppressed revolts among the Moors, and won from the senate the usual ceremonies of thanksgiving. The war with the Parthians had not at that time advanced beyond the preparatory stage, and Hadrian checked it with a personal conference.
After this Hadrian travelled by way of Asia and the islands to Greece, and, following the example of Hercules and Philip, had himself initiated into the Eleusinian mysteries. He bestowed many favors on the Athenians and sat as president of the public games. And during this stay in Greece care was taken, they say, that when Hadrian was present, none should come to a sacrifice armed, whereas, as a rule, many carried knives.
Afterwards he sailed to Sicily, and there he climbed Mount Aetna to see the sunrise, which is many-hued, they say, like the rainbow. Thence he returned to Rome, and from there he crossed over to Africa, where he showed many acts of kindness to the provinces. Hardly any emperor every travelled with such speed over so much territory.* The post above quotes the linked text, but combines information from the footnotes with selected passages from the translation of the Life of Hadrian to present a coherent account of his work on border maintenance.