About halfway up the image,is a dark band,dropping away,to the rhs.This is the rampart.Where it drops away,was the North gateway.Between the rampart and the front of the image,were 5 mounds and ditches,acting as deterrents.Beyond the fort,in the distance,near to the dark hedgerow,aerial photography has revealed,the existance of an amphitheatre,and other Roman buildings.
see http://www.dur.ac.uk/resources/archaeological.services/research_training/hadrianswall_research_framework/project_documents/Birrens.pdf
see http://www.roman-britain.org/places/blatobulgium.htm