We're talking about over 200 years after the Antonine Wall was abandoned. Certainly, at the time of the events in King Arthur, there was no military presence whatsoever. It's possible that some of the forts and fortlets may have retained a kind of use by the local populace, though it isn't likely they were still in an adequate condition. Collapsing masonry, rotting woodwork, incursion of the local flora, all that kind of thing, which would have been exacerbated by the traditional Scottish weather. There's a great video that plays at the top of the viewing tower at Segedunum (Wallsend) showing the decay of the fort over time, once abandoned to its fate.
The wall itself was a turf wall (probably topped by a wooden palisade and walkway). The wooden part would be long gone, and the wall would have been a shadow of its former self. Since the replica was built at Vindolanda in the 70s, it has lost over half a metre in height, and that is without allowing trees, bushes, etc. to get a foothold in it.)
Well spotted with the Avatar, by the way. The prize is a guided tour of Lindum Colonia (Lincoln) with an experienced, knowledgeable , witty, handsome and modest guide. Let me know the next time you're there and I'd be happy to show you its many Roman ruins.
Great information, I appreciate it. (I'm the kind of person who watches things and has to go look them up.)
I am putting Lincoln on my bucket list and if the economy ever turns and I can afford to travel again, I'll let you know when I'm coming. In the meantime if you want a guided tour of something with nice mountains and a lot of elk, but absolutely no Roman ruins whatsoever, let me know.