originally called llantilio castle, white castle was one of three fortifications built in the monnow valley around the same time, possibly by earl william himself, to protect the route from wales to hereford, and overlooked the manor of llantilio crossenny and the river monnow. the first castle on the site was built from earth and timber, with three large earthworks forming an inner and outer ward, and a hornwork protecting the main entrance to the south. a mill was constructed at great trerhew to grind corn for the castle garrison.