 Fairfax ordered the castle to be totally destroyed under the supervision of Henry Herbert , a descendant of William ap Thomas . The fortifications proved too strong , however , and only a few of the walls were destroyed , or slighted . Historian Matthew Johnson describes the event as having the atmosphere of a " community festival " , as local people dredged the castle moat in search of treasure , and emptied the fishponds of valuable carp . The castle 's library , including an important collection of Welsh documents and books , was either stolen or destroyed . 
