3. Hadrian then constructed the Vallum, as a limes or boundary, slightly to the south of these forts, yet everywhere north of the older line of Agricola.
4. Hadrian finally built the Great Wall, linking up the new forts and including a mile-castle every seven furlongs, and two wall-turrets between every pair of mile-castles. He also made a road from castle to castle, and so from fort to fort—122-127 A.D. This great Stone Wall, was the last word in the defensive problem. The original scheme, of forts and Vallum, had failed to ensure the safety of the frontier; and the necessity for a continuous barrier had become evident.
So it came about that the Wall was built. In the course of its building, the Vallum with its deep ditch and high mounds would form a continuous obstacle to the free passage of workers and building materials coming from the south. To obviate this difficulty, the mounds of the Vallum were cut through and the ditch was filled up at frequent intervals, to form temporary level-crossings.
5. After the Great Wall was completed, the Vallum-ditch was cleared where necessary, and the clearings from the ditch were thrown up as an additional mound on the south margin of the ditch. This indicates that the Vallum still represented a boundary: no longer, indeed, to the enemy, for the Wall was now their boundary, but to the civil population of the province of Britain. North of the Vallum was now a military district, "out of bounds" for civilians.
6. Severus reconstructed great portions of the Wall and forts, which had been thrown down by the enemy—207-210 A.D.
It is thus clear that most of the work which we see was originally designed by Hadrian in the second century.
It would serve no purpose for me to go into all the older arguments and theories as to who was the builder of the Wall, and what was the object of the Vallum, but I have endeavoured to give the latest views, based on the most recent discoveries.
As is well known, archæologists do not now aim at finding objects, but rather at learning history and fixing dates. Especially do they aim at dating the various levels of occupation by means of the pottery fragments found there—a method unheard-of when excavations were first begun on the Wall. Much still remains to be done in this direction.
The evidence in favour of Hadrian's having been the builder of the Wall is now so strong as to be irrefutable. In four mile-castles have been found slabs bearing his name and that of his proprætor, Aulus Platorius Nepos. The name of Severus has not been found at all on the actual line of the Wall.