Marking the north-western frontier of a once vast empire, Hadrian's Wall survives today as the greatest Roman monument to have been left in Britain. Construction of the wall was ordered by the Emperor Hadrian cAD120, nearly a century after the Roman conquest of the island. Running the complete length of the Tyne Solway isthmus, and covering a distance of some 73miles (117km), it took some six years for Hadrian's Wall to be completed.
Throughout its length the features were consistent, comprising of the wall itself with a defensive ditch on the northern side, and the spoil from the excavated ditch forming an earthen rampart along the northern edge. Gates and a small guardhouse were placed at mile intervals (referred to as 'milecastles'), and equidistantly between each milecastle two observation towers (known as 'turrets') were built. Originally conceived in two distinct sections, the building materials used for the western section to Solway comprised of turf blocks for the wall, and turf and timber for the Milecastles, with the turrets being the only stone structures. The other section, from modern Newcastle in the east to the River Irthing near modern Carlisle, was constructed entirely from stone. The western turf section of the wall was eventually rebuilt in stone, sometime in the late 2nd century.
Hadrian's Wall was probably designed originally more as a demarcation line rather than as a strong defensive barrier, because it would have been almost impossible to have had sufficient men available to defend a given point of this extensive frontier from a concerted enemy attack. However, as with most projects, changes were made during the course of construction, the most significant being the addition of large forts that allowed troops to be garrisoned actually on the wall. Originally twelve in number but eventually sixteen, wherever possible these forts would have been built astride the wall with three of their four gates opening out to the northern side. At the same time a second large ditch (Vallum) was dug to the south of the wall, the spoil from which was used to build an earthen rampart along both sides, having the effect of clearly defining the boundary of the military zone.
Remarkably this huge building project was abandoned when Hadrian died in AD138. His successor, Antoninus Pius, decided to move the frontier forward some 100 miles (160km) to the north, and a new turf wall was built along the Forth Clyde isthmus. Traces of the 'Antonine Wall' can still be clearly seen today. On the death of Antoninus Pius the succeeding joint ruling emperors preferred to re-occupy Hadrian's Wall, and during this period a military road was laid between the wall and the Vallum. This was intended to provide an easier means of communication between the wall forts.
For the next 250 years the history of Hadrian's Wall is dotted with uprisings and periods of peace. Upon withdrawal of the Roman forces from Britain in the early 5th century, the wall was finally abandoned. Virtually nothing is known from that time, except that much of the stone was quarried away for local building. Today, declared a World Heritage site since 1987, Hadrian's Wall is in various states of excavation, but one of the best surviving examples of the original wall can be seen lying between Sewingshields and Steel-Rigg, west of Housesteads Fort.