This ditch would have been the western facing defensive ditch and rampart.The west is to the lhs of the image.Curiously,the rampart continued over the edge of the steep drop in the trees at the top of the picture,before turning sharp right and contouring the slope.
see http://www.coflein.gov.uk/en/site/304294/images/CASTELL+GWYN,+LLANDISSILIO+WEST/