The building of Hadrian's Wall AD 122 - 139
After several attempts to conquer 'Scotland', the Romans withdrew in AD 105, and the northern frontier was temporarily fixed at the Tyne-Solway line. Emperor Hadrian visited Britain in AD 122, and decided to build a physical frontier along this line, which was completed in AD 139.
The wall was seventy-five miles long and two to three metres thick. Eighty mile-castles and forts were built along the wall, which was garrisoned by auxiliary troops. It was built ostensibly to separate the Roman province from the uncharted, barbarian land to the north but undoubtedly it also served as a powerful visual symbol of the might of Rome. The wall had castellated defences on both sides.
The Northern tribes continued to rebel, necessitating the refortifying of the wall numerous times.
http://www.bbc.co.uk/history/timelines/britain/rom_hadrians.shtml