.The Monnow Valley, down the right side of the sheet, forms a natural break in defences along the border of southern Wales.,Depicted along the valley are the Norman,castles of Grosmont, Skenfirth, Tregets and Monmouth, which,once guarded the routes between Herefordshire and Wales. ,Monmouth was of particular importance, lying exactly on the border where the river Monnow joins the Wye. Iron roads, the horse railroads pre-dating the locomotive era, feature at lower right. Highlighted,in ochre, these were probably added,to the plan some time after its official completion.
