The Castle of Otranto is a 1764 novel by Horace Walpole. It is generally regarded as the first gothic novel. In the second edition, Walpole applied the word 'Gothic' to the novel in the subtitle – "A Gothic Story". The novel merged medievalism and terror in a style that has endured ever since. The aesthetics of the book shaped modern-day gothic books, films, art, music and the goth subculture.

The Castle of Otranto was written in 1764 during Horace Walpole's tenure as MP for King's Lynn. Walpole was fascinated with medieval history, building in 1749 a fake gothic castle, Strawberry Hill House.

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, whom he feels has failed to bear him a proper heir.

However, as Manfred attempts to marry Isabella, she escapes to a church with the aid of a peasant named Theodore. Manfred orders Theodore's death while talking to the friar Jerome, who ensured Isabella's safety in the church. When Theodore removes his shirt to be killed, Jerome recognizes a marking below his shoulder and identifies Theodore as his own son. Jerome begs for his son's life, but Manfred says Jerome must either give up the princess or his son's life. They are interrupted by a trumpet and the entrance of knights from another kingdom who want to deliver Isabella. This leads the knights and Manfred to race to find Isabella.

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