Straddling the border between Herefordshire and Monmouthshire in the lee of the Black Mountains, the parish of Longtown has the remains of two castles. One is a popular visitor attraction in the care of English Heritage, with a stout-walled round keep. Half a mile away is another, in the form of a large earthen mound. When the members of the Longtown & District Historical Society decided to investigate, they realised that little was known about either castle, so they set out to find out when…