The fort at Stanwyck is a bit of a puzzle - it was the headquarters of the Brigantes (just) pre-Roman occupation. there is a long story involving a Queen and lust, a dissapointed husband etc. But apparently the southern gates were only hastily finished just before the Romans came.
The sheer size of the enclosure and its location makes me wonder if it wasn't really a fort but rather a fortified enclosed market for cattle. I may be wrong but I think that the trade in scots cattle was the major economy. Wool came later and was exported mainly through Berwick.

It would make sense if they left the southern gate until last if the purpose was pointed north rather than towards the south. This presumes a trade in cattle fom real antiquity - before the Romans.

I think that this is a subject that deserves much more research.