CATHERINE FISHER IS looking a little pale after spending an hour or so being photographed with the London Dungeon’s most grisly instruments of torture. You would not guess, unless you happened to have read her latest novel, Incarceron, that she has imagined much, much worse.

The ultimate prison, Incarceron not only imprisons its captives for ever, but devours and recycles them. So vast that it contains forests, cities and seas, it has been devised centuries ago as a place without hope – though a legend persists that one man managed to escape.

Finn is a boy without memory, surviving Incarceron with thieves who trick and murder other prisoners. Outside, beautiful Claudia is equally imprisoned in a society where everything is frozen in an idyllic version of the 17th century. She and Finn each discover a mysterious crystal key. The two prisoners – one on the inside, one on the outside – can use the keys to communicate, but can they escape?

Incarceron marks a particularly fruitful development for an author who was shortlisted for the Whitbread Children’s Prize in 2003. Fisher has been writing fantasy since 1990, and ever since her Snow-Walker’s Son trilogy (1994-96) has been one to watch both for her remarkable imagination and a pared-down prose style reminiscent of Ursula Le Guin and Alan Garner.

“Oh, of course Alan Garner is a huge influence,” she agrees. “His writing is so intense, he packs in so much, and then there’s the way he uses landscape . . .”

Her own writing, too, is steeped in the landscape of her native Wales. The daughter of a book-loving painter and decorator, she read English at the University of Wales in Caerleon, outside Newport, taught there, took part in a year-long archaeological dig in the town’s Roman ruins and still lives in the Wye Valley. Incarceron itself germinated from an exhibition in Cardiff of Piranesi’s imaginary prisons – “those vast, shadowy chasms and viaducts, with tiny people, which I’m sure also influenced Mervyn Peake’s Gormenghast”. Fisher’s sentient prison is an elaboration on many philosophical ideas concerning imprisonment, with a twist at the end that will lead to a sequel, Sapphique, next year.

A small, slight, private woman with long, wavy hair, she looks very much the seer, and writes her novels to “find things out”, rather than planning the plot. She creates unforgettable characters, such as the albino wizard Kari, the Snow-Walker’s Son, and the boy-god Alexos: children whose devastating gift of magic is matched by vulnerability, needing support from a cast of fierce heroines, shrewd bards and semi-corrupt officials. She is far from the sword and sorcery of cliché.

It has taken Fisher a surprisingly long time to break through to a more general readership. Her Oracle trilogy was optioned for film only after the Whitbread, an event that also caused her to be published at last in the US. She is plugged into the world of Welsh poetry but not of children’s fiction, although she enjoys it. Her imagination gets recharged by the Icelandic sagas and Welsh triads.

She takes pride in the Welsh bardic tradition, going back to Taliesin. In Corbenic (about a bitter, modern boy deserting his mentally ill mother) she reworks Arthurian legend. To me, her most interesting work is, like the four-volume Book of the Crow and Incarceron, closer to SF, imagining a place where science and magic overlap. As Le Guin advised in a famous essay on fantasy fiction, she thinks about the practical consequences of having two moons, or bad weather. You are made to feel the bitter cold through which Kari and his friends travel in The Snow-Walker’s Son, and the parched deserts of The Oracle. “All my places are real places,” she says. “There has to be a specific landscape.”

She is a Catholic and, though wary of going into this too deeply, says that “the sacramental idea that everything is charged” is as important to her as her feeling of “rootedness” in the Welsh landscape.

Although Incarceron is the ultimate prison, Finn is one of a long line of Fisher heroes who must work through imprisonment before finding liberation through truth. For all her sensitivity, her nerves are much stronger than my teenage daughter’s; when we go into the London Dungeon after the interview, we last precisely three minutes before running, terrified, for the exit. But if you’d read Incarceronbeforehand, you might do the same.

INCARCERON by Catherine Fisher
Hodder Children’s, £5.99; 464pp £5.69 (free p&p) 0870 1608080 timesonline.co.uk/booksfirst

Explore Books

• Book Extracts

• Book Reviews

• Books Group

Times Recommends

• Meet Perry Moore, caped crusader for gay superheroes
• Bolting off for a bit of white mischief
• Girls Like Us by Sheila Weller