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 they were built, by whom, and why there were two in such close proximity. Current Archaeology's Chris Catling reports.