The ridges and furrows,clearly visible,are the remains of the five defensive ditches and mounds protecting the northern aspect of the fort.River erosion,to the east and south,means that it's not possible to accurately determine if the whole fort was so well protected in this way.To the west of the fort was an annexe,roughly the same size as the fort,but with only a single ditch for defence.
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