One fact about Hadrian's Wall which always astounds me,is the placement of the Wall garrison's mile fortlets,or milecastles.They were placed,almost religiously,at exactly 1 Roman Mile apart;regardless of the terrain, and regardless of the daily use-ability of the location.They were supposed to be the general daily traffic portals/gateways through the Wall.In order that The Garrison could keep an eyeon/limit the trans wall movement/levy taxes on traffic as it crossed the barrier.But milecastles can be found atop cliffs,beside swampy ground,as can be seen in this shot,on very unsuitable ground for internal buildings.Look at the slope on the interior of this milecastle,then look at milecastle 42,at Cawfields.