From the image,just below the summit are a shadowed area and a darker area.Just below both these zones are the clear earthwork defences of a very large Roman camp and a Roman fortlet.Also visible just below the darker zones are three small mounds.Archaeolgical excavation has shown the large camp to have been a camp where the Romans practised seige warfare.The three small mounds are believed to have been support mounds for ballista catapults.The fortlet was Agricolan,circa80-86AD and so pre-dates the large camp.
see http://www.roman-britain.org/places/burnswark.htm
see http://canmoremapping.rcahms.gov.uk/index.php?action=do_advanced&idnumlink=66634
I'm no expert;I should have liked to be,but it didn't happen.There are miriad sites accorded to the near mythical Battle of Brunanburgh,so for a prestigious organisation such as RCHAMS to come out and state any location for said battle seems rash,in the extreme,yet,here was the place;so they claim.
see http://canmore.rcahms.gov.uk/en/site/72885/details/burnswark+roman+siegeworks/