What is the theme in The Mysteries of Udolpho by Ann Ward Radcliffe?

Supernatural occurrences repeatedly appear throughout the novel, but they are later explained rationally. Mysterious music plays at night at castle di Udolpho, but Emily learns that the music is played by Chevalier Du Pont. The repetitive voices that resound through the walls at castle di Udolpho also is revealed to be Chevalier Du Pont, as is the figure on the ramparts. The mysterious chamber next door to Emily's apartments at castle di Udolpho is rumored to be haunted. It is made more mysterious by Montoni's late night visits.