 In 1932 the Duchy gave custody of the castle to the Office of Works , while retaining ownership . The site is today cared for by English Heritage , the successor to the Office of Works . The surrounding landscape has been protected as a national park since 1951 . The castle is a scheduled monument , which means it is a " nationally important " historic building and archaeological site which has been given protection against unauthorised change . It is also a Grade I listed building ( first listed in 1985 ) , and recognised as an internationally important structure . It has been described as " perhaps the finest medieval landmark of the Peak District " , and architectural historian Nikolaus Pevsner remarked that it is " By far the most important castle in the county – in fact the only one of importance " . Before Duffield Castle in the south of the county was destroyed in 1266 it had one of the largest keeps in England , though only the foundations survive . 
