In 1988, James Cawthorne and Michael Moorcock included The Castle of Otranto, generally considered the first gothic novel, on their list of influential fantasy books. I mentioned this last year, but I just got around to reading the novel — and it really isn’t very good. In fact, it reads like a bad Shakespeare pastiche, full of star-crossed lovers, long-lost relatives, comic-relief servants, and a few apparitions.
I do believe Walpole did introduce one now-cliché trope though: the door that opens for no apparent reason, followed by a sudden draft that snuffs out the light.
Actually, a quick search reveals — brace yourself — a TV Tropes page devoted to the novel, another describing the genre and naming its prominent authors, and another listing all the genre’s tropes. Apparently they consider Otranto the trope-codifier for the haunted castle.