In the introduction the unique character of Valkenburg castle is pointed out in comparison with other castles and ruins in the Netherlands. As it is an elevated castle (rare for the Netherlands) it does not fit in with the typo-chronological outline of H.L. Janssen. Because of the complicated building history of Valkenburg castle with its numerous building periods, it takes up a special place in Dutch building and architectural history. A consolidation campaign in progress at the moment offered a good opportunity to pay renewed attention to the large changes the ruin went through during the past 150 years. The consolidation periods will be investigated, the accompanying restoration ethics and the building-historical and archaeological resea...