The novel, which initiated a literary genre that would become extremely popular in the later 18th and early 19th century has been translated into Persian by Kaveh Mir-Abbasi. Tehran-based Ofogh Publishing has released 'The Castle of Otranto' in 152 pages.

Set in a haunted castle, the novel merged medievalism and terror in a style that has endured ever since. The aesthetic of the book has shaped modern-day gothic books, films, art, music, and the goth subculture.

Walpole was inspired to write the story after a nightmare he had at his Gothic Revival home, Strawberry Hill House, in Twickenham, southwest London. Claiming he saw a ghost in the nightmare—which featured a "gigantic hand in armour"—Walpole incorporated imagery from this into the novel, and also drew on his knowledge of medieval history.

'The Castle of Otranto' tells the story of Manfred, lord of the castle, and his family. The book begins on the wedding day of his sickly son Conrad and princess Isabella. Shortly before the wedding, however, Conrad is crushed to death by a gigantic helmet that falls on him from above.

This inexplicable event is particularly ominous in light of an ancient prophecy, "that the castle and lordship of Otranto should pass from the present family, whenever the real owner should be grown too large to inhabit it."

Manfred, terrified that Conrad's death signals the beginning of the end for his line, resolves to avert destruction by marrying Isabella himself while divorcing his current wife, Hippolita, who he feels has failed to bear him a proper heir in light of the sickly condition of Conrad before his untimely death...

'The Castle of Otranto' is the first supernatural English novel. It blends elements of realist fiction with the supernatural and fantastical, establishing many of the plot devices and character types that would become typical of the Gothic novel: secret passages, clanging trapdoors, pictures beginning to move, and doors closing by themselves.

The poet Thomas Gray told Walpole that the novel made "some of us cry a little, and all in general afraid to go to bed o’nights."