I was thinking about this too and how common and cheesey it sounds to us now because we live in a generation where horror films are on the cinema screens at least twice sometimes ten times every year and are in every movie rental shop. For each new generation it seems to take more to make our hair stand on end and thrill us to the core!
In the middle of the chapter, when Isabella is going through the caverns, I was really into the story, and I do believe I had goosebumps on my arms when Isabella and the peasant heard Manfred comming toward them.
This type of story may seem silly to use now but I suppose it really was very thrilling to those who had lived a fairly sheltered life like Catherine Morland or even Isabella Thorpe and remeber even Henry Tilney, that sensible witty young man, said his hair stood on end when he read 'Udolpho'! :)