Considering that the Priddy landscape is pock-marked with lead workings dating from the roman to the medieval periods, it's not an unreasonable supposition that merchants from around the roman world visited the area.
I found a reference to Gorsey Bigbury near Longwood Farm in the parish of Charterhouse which has a similar enclosure 160 ft across ditch inside the bank & one entrance with a horseshoe setting of postholes within.