 When John died , Edward III reclaimed Berkhamsted Castle ; a survey showed it to be in need of substantial repairs . Edward had not yet improved Windsor Castle , so used Berkhamsted as his main property , investing considerable sums in renovating it . His son , Edward , the Black Prince , was created Duke of Cornwall and also made extensive use of the castle , which formed part of the new duchy . The Black Prince took advantage of the aftermath of the Black Death to extend the castle 's park by 65 acres ( 26 ha ) , including some woodland pasture stretching over the Chilterns , eventually producing a park covering 991 acres ( 401 ha ) . The castle was used to hold John II of France after he was taken prisoner at the Battle of Poitiers . When the Black Prince fell ill following his campaigning in France , he retired to Berkhamsted and died there in 1376 . 
