It’s hard to resist, and every child has to do it. As they cross the articulated wooden walkways suspended 60ft above the ground they just have to jump up and down, making the bridges swing and their legs wobble. For those worried by heights it is a challenging moment to say the least.

The walkways lead to one of the world’s largest tree houses - a series of turret-topped fairy-tale cottages - the whole structure built into the higher branches of 16 lime trees with breathtaking views across Northumberland. The tree house moves and bends as the trees grow or rock in the wind. Below is a wooded ravine, soon to be the site of Europe’s largest adventure playground; beyond is the expansive, romantic Capability Brown landscape of Alnwick Castle.

The tree house and the play ground are the latest additions to the Alnwick garden, which opened in 2002 and now attracts more than half a million visitors a year, and which is forging a radical agenda on play.

Wheelchair users can go up through the tree canopy, and will be able to access the adventure playground. The purpose of these ambitious projects - the tree house cost pound;3 million, the playground a “substantial” sum, paid for with a mixture of private and public funding and with a lottery bid pending - is to expose children to risk-taking and challenge in an environment that promotes imaginative freedom.

Although the garden has conventional flowerbeds, it also has a poison garden full of exotic and toxic plants. There’s a bamboo labyrinth where children can hide and explore imaginary worlds, and a vast number of sculptural waterfalls. At any Alnwick play event you will find children getting wet running through water jets or standing under water spray, letting it cascade over hands and feet.

Alison Hamer, Alnwick’s learning programmes manager, says the garden’s trustees took to heart the premise that children develop by taking risks and learn through their senses. “We like children to get wet and dirty, to face physical challenges in order to promote mental development. This is a no-worksheet garden. We have wrapped children up in cotton wool for too long; you can break an arm tripping up over a stone. There has to be risk-taking.”

The adventure playground, still at the design stage, is intended to be groundbreaking, not only in scale - it will accommodate 1,000 youngsters at any one time - but in its scope for giving children of all abilities the freedom to climb, tunnel and dig; abseil; muck around in hollows and sand banks; and indulge in make-believe and role play.

The idea for the garden came when the Duchess of Northumberland, whose home is Alnwick Castle, was out walking with her five-year-old son Max, one of her four children. She watched him climb a tree in a wilderness area of the estate overlooking beech trees, meadows and the river Aln and imagined transforming this wilderness into a place where other children could come and have the freedom to climb and explore.

The Alnwick venture is as far away as you can get from the usual play park, with swings and multi-coloured climbing frames which, according to a Children’s Society and Children’s Play Council survey, many children find boring. (The council recently received government funding to look at ways to expand play opportunities in England.) The Duchess has asked Gunter Beltzig, who designed the pirate ship for the Princess Diana playground in Kensington Gardens, which has given London’s children hours of imaginative free-for-all, to design the Alnwick playground. His aim, he says, is to “open the valve” to children’s “joy of playing” and their “creativity and spontaneity” in an “ever-changing world of uncompromising dreams and wishes”. The best design, he believes, is one that can respond to the “unplanned for, different use it is put to”.

Beltzig will be assisted by Brummie Stokes, a consultant for the treehouse project, who runs an outdoor activity centre on the Welsh borders called A Taste for Adventure. Stokes, an ex-SAS soldier and mountaineer who lost his toes to frostbite on one of many climbs up Everest, is noted for his ability to inspire disabled children to face physical risk and challenge.

He grew up in poverty in the back streets of Birmingham where canal banks and waste ground were his childhood haunts, and at the age of nine stowed away on a Glasgow-bound train. “My life then was full of dangers,” he says, “and I’m not suggesting children should be exposed to such danger, but there is a human need for excitement and adventure, to push at barriers, and if young people cannot access it then they will seek it in anti-social behaviour.

“Play activity, if it is challenging enough, will fulfil that need for excitement in a positive way. If we don’t have such opportunities then we end up with kids with lots of adrenalin, with nowhere to go and no ability to manage risks.”

www.alnwickgarden.com