The building of some 75 miles of massive curtain-wall, incorporating forts, milecastles, turrets and bridges, together with a further chain of forts, milefortlets, and turrets 32 miles long on the Cumbrian coast, represents an impressive civil-engineering undertaking by any standards. The fact that this was accomplished in a relatively short space of time on the remote northern fringe of the Roman Empire makes the achievement even more remarkable. Unfortunately, there is a lack of firm evidence for many aspects of the building of the Wall, so that it is impossible to assess precisely the scale of effort and organisational complexity involved in this operation. Many details of its original structure remain a matter for conjecture, since it has nowhere retained anything like its original height, and large sections of its length lie beneath buildings or roads, or have been destroyed by quarrying, cultivation, or the robbing of stone for construction. Alterations during the three centuries of Roman activity on the Wall have, in many cases, obliterated evidence of the earliest structural features. The actual source of much of the original building material is uncertain, so that transport routes and distances must remain a matter for debate. There is little archaeological record of Roman transport methods, and depictions of vehicles, such as those on Trajan's Column, owe more to artistic licence than to realistic detail, so that here again methods can only be conjectured. Literary record of happenings in Britain around A.D. 120 is sparse and precise dating of events, such as Hadrian's presumed visit to Britain, rests largely on a concensus of opinion based on circumstantial evidence.