The American author Louis Sachar’s most celebrated book, 1998’s YA novel Holes, was a huge word-of-mouth success on both sides of the Atlantic. Its short, punchy chapters tell the story of plump, hapless Stanley Yelnats, sent to a summer camp for wayward boys, where a terrifying Warden has peculiar ideas about character reformation. The 5ft-deep holes the boys are required to dig turn out to have a surprising purpose. Shifts of time, register and perspective render a simple premise mesmerisingly intricate. It has peril, love, crime, wickedness, redemption and friendship in, well, spades.

A quarter of a century later, Sachar has written his first supposedly adult novel, in which many of the same ingredients reappear. A man “dressed like a typical American tourist”, but with an odd habit of storing cake crumbs in the pocket of his hoodie, has arrived at the castle of the title, filled with curiosity to see how much has changed in the past 500 years. He seems to know intimate details about daily life back then in the court of King Sandro, Queen Corinna and the headstrong teenage princess, Tullia. Anatole, the king’s bumbling magician and alchemist, was fast losing prestige due to his abject failure to turn black sand, brought in from Iceland at huge expense, into gold. But the magician evidently achieved one stunning success, for Anatole is our present-day narrator. Grisly legends have built up around the castle, as eagerly related by the tour guide, but are full of errors. Anatole decides to recount the real story.

This former contemporary and rival of Leonardo da Vinci, now an immortal wanderer, explains how the near-bankrupt kingdom of Esquaveta depended for survival on a dynastic marriage between Princess Tullia and Prince Dalrympl of wealthy Oxatania; his betrothal gift of a tiger gave the castle its name. But Tullia loved Pito, a young scribe. Incensed, Dalrympl demanded that Pito be beheaded during the wedding feast. Anatole was tasked with ensuring the royal wedding went ahead and thus saving the realm. Timid and compromised, he brewed various bizarre concoctions supposed to inspire love and delete memories, but became embroiled in the lovers’ situation, especially after recognising Dalrympl as the swine who caused the death of Babette, his own sweetheart.

The portrayal of science in the period is lightly comical but never patronising. Although he can’t conceive of a building block of matter any smaller than a grain of sand, Renaissance-era Anatole is almost on the point of discovering penicillin via his experiments with mouldy bread. In the present day he has centuries of scientific discovery to call upon; all the same, he muses, is the notion of the four humours any more fantastical a metaphor than the periodic table? Operating at the birth of modern science, Anatole had it both ways, creating effective medicines while also commanding respect as a controller of evil spirits.

Despite a daring escape, pursuit and the threat of retribution, the air of whimsy throughout mitigates any sense of genuine peril (and we obviously know Anatole survives). Just as in Holes, Sachar knots plot threads together in unexpected and satisfying ways. There’s a dash of Patrick Süskind’s Perfume in Anatole’s experiments, but without that novel’s sombre heft. If the narration were given over to Tullia or Pito, this would be a standard YA novel; but then we’d be without the pleasures of Anatole’s long view and his gentle scepticism about love, war, human nature and politics.