Built on the orders of Roman Emperor Hadrian, the wall was constructed from about 122 AD until about 128 AD. The wall stretched 73 miles (80 Roman miles) between the North Sea and the Irish Sea. It was actually 5 metres high (15 feet) - in some places augmented by a north-facing parapet wall another 1.5 meters high for the wall's sentries to take shelter behind. The overall effect was magnified by the deep V-shaped ditch dug along the north face of the wall - a classic Roman infantry defense.
A small guard tower known as a milecastle was located every Roman mile along the wall, staffed by a small garrison unit of 8 to 10 men. Two smaller sentry turrets were situated between each milecastle. Gates in the wall were guarded by larger fortifications.
A recent (November 2000) discovery of a wall section near Newcastle provided evidence of three rows of sharpened, interlaced stakes between the Wall and the northern ditch. These spikes are thought to be the Roman version of modern-day barbed wire. (See http://www.independent.co.uk/news/UK/This_Britain/2000-11/wall021100.shtml for this story.)
South of the Wall was a Roman road built for military use, and then another, shallower ditch, called the Vallum. This ditch, and the mounds of earth that lined it like riverbanks, served to keep civilians and their livestock from venturing too close to the Wall. It also served to discourage the rebellious Brigantes tribes from getting friendly with their northern neighbours.
A more northerly earthwork defense, the Antonine Wall, actually marks the "high tide" or Roman influence in the British Isles, but Hadrian's Wall was the limit of the Roman empire for most of the Roman era in Britain.
The Roman hold on Britain began to lapse, as troops were called away from Britain to combat the Vandals and Goths. A massed force of barbarians including Picts, Scots and Saxons overwhelmed the Wall's defenders in 367, and despite a later attempt to reassert Roman control, by the year 400 the Wall had been abandoned. A stirring visualization of the Wall's failure in the barbarian invasion can be found in Jack Whyte's novel The Skystone, the first volume in the A Dream of Eagles cycle.
Much of the Wall was dismantled for building materials during the subsequent centuries, and very little of the Wall itself remains standing today. Parts of the Wall remain in the Northumberland area, and some of the milecastles and smaller forts also remain. Hadrian's Wall was designated as a World Heritage Site in 1987.
Sources include http://www.hadrians-wall.org/ and
http://www.britainexpress.com/History/Hadrian's_Wall.htm