In this image,can clearly be seen a ditch running,from bottom right into the middle of the shot,where the curve of the ditch takes it out of view,to a point just infront of that single tree.On the rhs of the ditch is a raised bank.The ditch is much shallower than in it's original form,weathering will have silted it up down the centuries.
see http://www.coflein.gov.uk/en/site/304294/images/CASTELL+GWYN,+LLANDISSILIO+WEST/