Gardening: Four acres that bring to life a private fantasy are open to the public. By Patricia Cleveland-Peck
The English tend to love follies, those small, decorative buildings constructed simply for enjoyment. It was particularly during the 18th century that thousands were put up on great estates such as Stourhead in Wiltshire and Hawkstone Park in Shropshire.

But follies are not solely the preserve of the past - they have become a passion for Colin Armstrong, the owner of Tupgill Park in North Yorkshire. Here, in collaboration with an architect, Malcolm Tempest, he has spent the past 10 years assembling an extraordinary collection within a four- acre garden that has become a maze known as the Forbidden Corner.

"It started as a private fantasy," Mr Armstrong says. "When the children were small they used to run round paths in a little wood playing hide- and-seek. Then we got hold of a strimmer in order to clear the paths, and started making a sort of maze that got more and more complicated."

When Mr Armstrong inherited Tupgill from his father, a racehorse trainer, there were a number of dilapidated stables there. "I called in Malcolm and mentioned that I'd like to make the area more interesting." The architect's response was to suggest a grotto - and thinking that a grotto just meant piling a few stones up, Mr Armstrong gave him a free hand.

Well, there is a grotto, known as the Underworld, at the heart of the Forbidden Corner, but now, some 50 constructions and many thousands of pounds later, getting to it is by no means straightforward. The whole folly garden has, in fact, been designed as a labyrinth of tunnels, chambers and surprises to make the quest as challenging as possible.

Entering by the Diabolical Gate, you set off down narrow, yew-lined paths through darkly Gothic shrubberies; you continue past a huge pyramid made of green translucent glass, and step up into an open, sunny garden. The entrance to the Temple of the Underworld seems near at hand but the only available path takes you away from it - into an opening known as the Eye of the Needle, where the tunnel narrows alarmingly, giving you the Alice-in-Wonderlandish experience of being far too big for your surroundings.

At every turn there are decisions to make and tricks to avoid. Most people, on seeing the inscription "Cave Aquae", cannot resist peeping into the "cave" - by which time it is too late to heed the warning that will have kept classicists dry. A little later, a couple of cunningly arranged steps invite you to climb up and look out at the view through a peep-hole in the wall - only to be subjected to another drenching. So, when, a few yards farther on, you see similar steps, you vow not to be taken for a fool again - but in fact this time there is no water, and no view: what looks like a window is, in fact, a mirror reflecting the fool after all.

A griffin, a very surprising horse, several dead-ends, a moving floor, ghostly voices ... to give away too much would spoil the fun. Suffice it to say that in one visit you will not discover everything the folly garden has to offer.

Mr Armstrong's private fantasy has now developed into one of the oddest and most engaging gardens open to visitors. In fact, the decision to open to the public last year was taken only after a family fancy-dress party in the grotto. "Many of the guests wanted to bring to bring their friends back to see the place," says Mr Armstrong. "So what began as a project for my future grandchildren has ended up rather differently - as, of course, once we'd decided to go public we had to put extra things in for the visitors."

Not everyone will approve. Some of the effigies of beasts and men would appear distinctly garish to an 18th-century folly-builder, and the poet who wrote the little verses inscribed on many of the structures was no Alexander Pope. Yet what saves the Forbidden Corner from the charge of being too like a theme park is the quality of the building. Mr Armstrong is lucky to have not only hundreds of tons of excellent stone on his doorstep, but also the services of a master craftsman, Denis Fawcett.

Great care has also been taken with the horticultural element of the project. The walled site includes several gardens proper, each reflecting a specific mood and providing an area for visitors to sit and reflect. These areas are managed by the appropriately named Tulip Bemrose, who is gradually filling in the woodland area with hardy, shade-tolerant plants and evergreens such as vinca, ivy, Garrya elliptica, Viburnum rhytidophyllum, box and yew, which reinforce the sombre mood. Her aim is to screen individual areas, so increasing the sense of mystery by preventing visitors from seeing too much at a time. Surprise, after all, is a big element in Mr Armstrong's garden.

The Forbidden Corner is at Tupgill Park, Coverham, Middleham, North Yorkshire. It is open on Sundays and Bank Holidays between March and October, from noon to 5pm - adults pounds 3.50, concessions pounds 2. For bookings and other opening hours, call 01969 640638.

Both Colin Armstrong and Malcolm Tempest are keen members of the Folly Fellowship, an organisation that provides information, publishes a magazine and arranges outings to follies. For details, write to it at 21 Beacon Road, Ware, Hertfordshire SG12 7HY.

Anna Pavord is on holiday.