The oldest building in the park is Ashtown Castle, a restored medieval tower house dating from the 15th century. Restoration began in 1989 and it is located beside the Visitor Centre which houses interpretive displays on the 5,500 years of park and area history. It was found hidden within the walls of a much larger and more recent building that was being used by the Papal Nuncio until 1978. At that time, the more recent buildings were deemed structurally irreparable due to dry rot. But as that was being demolished, Ashtown castle was re-discovered. Ornamental hedges show the footprint and plan of the house that surrounded it.