Hadrian's Wall
a good view of the wall and the dtich/vallum
The wall is only part of the defence system,though -- the ditch and long vallum (earthwords) were an integral part of the line. The stone parts of the wall have been recontructed in many places, but the stone was obviously used for buildings nearby -- cannabilized for stone work like nearly every other ruin was.
The wall was mostly abandoned after Hadrian's death, when the new emperror decided to move the northern border of the empire a few hundred kilometers further up -- building the turf-and-ditch Antonine wall in about 138 CE. The Antonine wall was quite a bit shorter, but had more forts along its length. The Romans fell back to Hadrians wall again after a failed attempt by Antoninus to conquer the northern tribes, and held Hadrian as the northern border until they gave up entirely on conquering Britain and left im the fifth century.
faint remains of the stone and dtich, looking west