Although King Arthur is predominantly associated with Southwest England (Cornwall, Somerset), there are a lot of northern legends, especially when taking into account the famous '12 battles'. There were definitely
campaigns in the Scottish lowlands above Hadrian's Wall against the Picts, especially
around Montrose, or Trimontium (the Eildon Hills). Birdoswald was a fort on the Wall
itself, on a magnificent site -- a bluff in a loop of the river Irthing. Its Roman name was
Camboglanna, and it could very well have been a base for one of Arthur's northern
expeditions. Some have suggested that this was the site of the Battle of Camlann (because of the 'Cam...'?), but this is unlikely.
Little remains of the fort, except for the enclosing wall, now no more than 4-6 feet high, which now contains a large farm. It would seem from the picture that the fort would be hard to defend, since it faces an upward slope. However, what you can't see is the steep drop-off to the river beyond the cattle meadow. Also, Hadrian's Wall itself ran along the front on the other side of the farmhouse.