Most scheduled monuments are earthwork structures and unoccupied historic buildings, usually in ruined condition such as castles, medieval abbeys and abandoned industrial remains.
Mae’r rhan fwyaf o henebion cofrestredig yn strwythurau cloddwaith ac adeiladau hanesyddol gwag, sydd fel arfer wedi dadfeilio, megis cestyll, abatai canoloesol ac olion diwydiannol.