Caer Penrhos is constructed on a knoll which rises above the main ridge on the S. of the Wyre. The ground slopes away in all directions from the earthwork, which consists of a single fosse and vallum. In the south-east is a large motte in the vicinity of which a ditch has been excavated from solid rock. This looks an example of a pre- existing earthwork having been adapted at a later period to form a motte and bailey type. The position commands a good view of the coastal strip at Llanrhystyd.