What does a haunted castle look like? Perhaps you imagine a ramshackle or imposing structure with towers, pinnacles and undefined spaces hidden in the shadows; some of its “prickly” gothic details picked out by moonlight. Threateningly ill-defined buildings, enough for our imagination to fill in the gaps, seem to be the ideal catalyst to spark terror.Ever since the publication of Horace Walpole’s The Castle of Otranto on Christmas Eve 1764, the novel has inspired visions of dark corridors, towering staircases, subterranean passages and moonlit chapels. The first novel to be identified as “gothic”, it’s presented as an ancient tale translated by the fictional William Marshal of Ghent. Portrait of Horace Walpole by Sir Joshua Reynolds (1756). National Portrait Gallery Otranto cemented many of the themes that came to define the gothic literary genre: supernatural apparitions, family curses, ancient prophecies and mystery. Even so, one of the novel’s greatest achievements lies in an obscure character hidden in plain sight: the castle itself.Woven throughout Otranto’s pages is a remarkable, changing architectural setting playing host to the novel’s action. The building, however, remains elusive. You cannot draw a map of the castle because it is not mapped out in sufficient detail. This is despite Walpole, in the guise of Marshal, teasing readers in his preface to the first edition that it is “undoubtedly laid in some real castle”. He writes: The author seems frequently, without design, to describe particular parts. The chamber, says he, on the right-hand; the door on the left-hand; the distance from the chapel to Conrad’s apartment: These and other passages are strong presumptions that the author had some certain building in his eye. Perhaps it describes Walpole’s very own romantically styled gothic “castle”, really a country villa, in well-to-do Twickenham. Built across decades from 1749, Strawberry Hill recreated the architecture and sense of history found in Britain’s medieval cathedrals and homes. Strawberry Hill did not provide Otranto’s specific details. Instead, it helped shape the castle’s atmosphere: its dark passages, stained glass and carefully orchestrated contrasts of light and shadow embody what Walpole called “gloomth”. This effect is certainly echoed in the novel’s castle setting, yet Otranto is not Strawberry Hill in disguise as often suggested. The novel’s architecture is stranger, less fixed and far more elusive than any real building.Capturing the castleReaders encounter Otranto’s architecture by degrees. Walpole never provides a complete description of the building nor its exact appearance. Instead, the castle emerges in fragments, glimpsed through moments of fear, wonder and supernatural disruption. By refusing to pin down Otranto’s appearance and complete geography, Walpole encouraged generations of readers, artists and architects to imagine the castle for themselves. Strawberry Hill House in 2012. Chiswick Chap, CC BY Many drawings and paintings illustrating Otranto’s scenes were produced by fans of the novel. Those Walpole celebrated the most were produced in 1796, more than 30 years after the novel’s first publication, by teenage artist Bertie Greatheed Jr. Pasted into his personal copy of Otranto and today housed at Yale University’s Lewis Walpole Library in Connecticut, Walpole considered these watercolours the finest illustrations of his gothic story. He praised their ability to capture emotions and supernatural effects. On February 22 1796, Walpole wrote to the artist’s father.I have seen many drawings and prints made from my idle […] not one of them approached to any one of your son’s four – a clear proof of which is, that not one of the rest satisfied the author’s ideas – It is as strictly, and upon my honour, true, that your son’s conception of some of the passions has improved them, and added more expression than I myself had formed in my own mind; for example, in the figure of the ghost in the chapel, to whose hollow sockets your son has given an air of reproachful anger, and to the whole turn of his person, dignity. Scenes from The Castle of Otranto illustrated by Bertie Greatheed in 1796. The Lewis Walpole Library, Yale University Beyond the figures’ deep psychological intensity, what makes these drawings so compelling is Greatheed’s use of tangible architectural settings. Rather than serving as passive gothic backdrops, the spaces actively drive the drama. Pointed arches frame terror and underline monstrous acts, vast interiors dwarf the figures and deep shadows hint at hidden dangers. By grounding the supernatural in believable, relatable architecture, Greatheed amplifies the emotional impact of each scene.This relationship between architecture and fear lies at the heart of gothic fiction. Walpole believed medieval architecture possessed a unique capacity to stimulate the imagination. Gothic buildings, with their dramatic contrasts of light and dark, their hidden corners and soaring spaces, encouraged emotional responses that more orderly classical architecture rarely inspired. They could evoke awe, wonder, melancholy and terror.Each Halloween, gothic architecture returns to popular culture in the form of haunted houses, castles, abbeys and ruins. Yet Walpole’s Castle of Otranto reminds us that the most unsettling gothic buildings are often those we cannot fully see. Like Greatheed’s illustrations, they leave enough hidden in shadow for the imagination to complete the picture.This article features references to books that have been included for editorial reasons, and may contain links to bookshop.org. If you click on one of the links and go on to buy something, The Conversation UK may earn a commission.Peter Nelson Lindfield received funding from Yale University's the Lewis Walpole Library.