In AD 117 Roman Emperor Hadrian came to power and sought to increase security of the empire. He ordered the building of a wall across northern Britain. He came to inspect its progress in AD 122 (http://hadrianswallcountry.co.uk/hadrians-wall/hadrian%E2%80%99s-wall-big-history). The wall would stretch 84 miles from the Irish Sea to the North Sea, and would have mile castles or small forts.
We visited the extensive ruins of the Roman fort at Housesteads (http://hadrianswallcountry.co.uk/visit/housesteads-roman-fort), and also the ruins of another fort along the wall at Birdoswald.