The mood of Otranto is a vivacious combination of feverish excitement and sensational horror. As Walpole says in the Second Edition Preface, his intention was to keep up a "constant vicissitude of interesting passions" for his reader. The novel is a rollercoaster of emotions, which keeps the reader in a persistent state of fear and suspense.
At the book's peaks of dramatic interest, the reader is swept along in the extreme disappointment, shock, or happiness of Walpole's characters. This is at least partially because the consequences these people reap for their actions don't always seem justified by what they have done. It's difficult to predict where the emotional arc of the story is going when characters make decisions in the hasty and intense way that Manfred, Isabella, and their companions do. Walpole's reader feels breathless and pent-up as drama stacks upon melodramatic drama. The way the book incorporates supernatural and impossible events also contributes to this; who could have expected that the young heir Conrad would be bloodily squashed by a giant helmet, or that this would lead to Manfred attempting to rape his dead son's fiancée? Nobody is safe, whether they're inside or outside the castle's walls.
Interspersed with this frightening and dark mood, however, are regular moments of humor. A reader mostly finds these in The Castle of Otranto when Walpole is satirizing or parodying elements of medieval literature for their entertainment. There are also instances where servants and underlings do silly or unexpected things and make snarky comments. These lighten the mood, creating a contrast with the darker instances that inevitably follow. Overall, the mood of this novel is unusually variable, but always intense and impassioned.
