Above may be seen a facsimile of the type of shutter which was used in medieval times to guard the bowman against a return flight of arrows. The Abbey wall which is 13th and 14th century is unique in that its battlements retain the grooves for these shutters which swing on trunnions the wooden guard after the bowman had fired his arrows in quick succession was swung down to protect him. The grooves do not exist anywhere else on the City walls and it is doubtful if there are any others in England except possibly at Alnwick.