The ruined mountain fortress town of Kuelap in Peru is Mark Mann's romantic secret garden
A remote ridgetop 10,000 feet up in the Andes. A stone watchtower rises from the dawn mist that cloaks a ruined city. I stand on the earth- filled tower, perched on the cliff edge, and gaze out over cloud-obscured valleys at waves of distant brown hills. I feel like an explorer.

But this is northern Peru, not the south: this city in the clouds is not Machu Picchu, but Kuelap. It is perhaps the most imposing ruin in South America, but the chances are you've never heard of it. Few people have. Tourists descend on Machu Picchu by the trainload, but I have Kuelap entirely to myself until two Swiss backpackers arrive at midday.

Both sites command spectacular hill-top positions, but while Machu Picchu is neatly maintained in the middle of forested mountains, at Kuelap the opposite is true. The land in front of the ruin is cultivated, a gently- sloping plateau suspended on a clifftop overlooking the 3,000-foot deep canyon of the Utcubamba river - a steep four-hour climb from the riverside village of Tingo, or two hours by road.

Reaching the plateau, the gentler final approach runs along a path flanked by maize fields and farmhouses. Chickens peck away in the dust and little boys in baseball caps skip to school, oblivious of Kuelap's massive walls dominating the ridge above.

Yet once inside these giant walls I feel I've stumbled into a romantic secret garden. A sudden explosion of wild cloudforest envelops the crumbling ruins of hundreds of small circular buildings. Scarlet bromeliads - air- plants that wrap their roots round tree branches - perch overhead, the size of footballs.

The site slopes, with different levels linked by short stairways. Inner walls enclose the highest section, maybe earlier defences that the city outgrew. Two men, whose only tools appear to be a blunt machete and a wheelbarrow, wage a hopeless war against the ever-encroaching vegetation.

Kuelap is almost totally unrestored, but comparisons with photographs of the unrestored Machu Picchu (or Maya ruins like Tikal) show it is in remarkable condition. The overgrowth and morning mist reinforce the air of mystery.

Historian John Hemming claimed this to be the largest pre-Colombian structure in all South America. He wrote: "Of all the myriad ruins in Peru, [Kuelap is] the most spectacularly defended, the strongest by European standards of fortification."

Stretching 600 yards along its ridge (its widest point is 110 yards across), the back overhangs an almost-sheer cliff. In front, overlooking the fields, are the mighty 60-foot high walls, backfilled with earth to make breaching impossible. The only entrances are three narrow slits. Each leads into a long, high-walled corridor that rises steeply and narrows until only one person can squeeze through. Attackers entering these corridors would have been easily picked off from the ramparts above.

Kuelap is reached from Tingo, itself two hours' drive from Chachapoyas, the sleepy "capital" of the province of Amazonas. The road follows the Utcubamba river through its impressive canyon, flanked by towering cliffs. I travelled on the back of a truck, sitting on a sack that contained two sheep and a chicken, all alive.

Despite its name, Amazonas is not jungle, but a temperate region of deep gorges on the eastern slopes of the Andes, below the high peaks but above the jungle. The valley bottoms are sub-tropical and the hillsides topped with cloudforest, full of hummingbirds, orchids and bromeliads and known as la ceja de selva - "the eyebrow of the jungle".

It's a remote, isolated region. To the east lies impenetrable rainforest. To the west, separating it from the main Andean highlands, is the vast canyon of the Maranon, the Amazon's greatest tributary. The weekly bus takes more than 20 hours to negotiate the hair-raising 140-mile route across the Maranon canyon to Cajamarca.

In the last few years it has become more accessible, if still little- visited. A less scary road has been built to the coast: although the route is three times longer, it only takes half the time of the Cajamarca route. Or you can fly from Lima.

Descending the worn steps of the watchtower, I reflected that if Kuelap was in the United States or Europe, there would be tourists swarming all over it and millions pumped into restoring and researching it. Then I noticed one half of the two-man maintenance team hacking away at some overgrown roots with his blunt machete, and I reminded myself of something.

This was Peru.