It's a strange looking place. The circumference seems too small for the height of the earthwork on the west side. You get the impression that they were once much higher all around, in which case it would have been quite an imposing structure, especially with a pallisade on top. Apparently there have never been any excavations, so it remains undated.
The bloke at Highburn House campsite can be petitioned for access permission.
It's interesting that Green Castle is also known locally as 'Cup and Saucer Camp' When considering that 'Cups and Saucers' is a colloquial term used in 19thC Northumberland for cup and ring marks, and that a marked stone was found in the ramparts of this site, it's possible that the local name refers not to the shape of the earthworks, but to rock art.