The Roman fort at Camboglanna on Hadrian's Wall is a candidate for the final battle of King Arthur at Camlann. The Roman fort of Camboglanna, also known as Birdoswald, is on Hadrian's wall about 30 minutes from the city of Carlisle. The south side of Camboglanna is above the River Irthing, with a steep drop down to the river. This could be the "crooked river" implied by the name.
However apart from a similarity of name of Camboglanna with Camlann, there is precious little else to suggest an King Arthur connection. Mordred plays an key role in the battle, and he is earliest mentioned with Arthur in Welsh Texts, which place the battle in Cornwall. Since then, the writings of Geoffrey of Monmouth, Wace, and others has kept the battle's location in the south of England, if not in Cornwall.
Camboglanna seems an unlikely candidate for Camlann.
Northern England Arthurian Sites