Once it was a ruin. Now it's a beautiful garden.

This is the story of one of Italy's most beautiful gardens - how it almost disappeared and was rescued by women of three successive generations who fell in love with it. It's also the story of a man who was chosen as a boy to guide the garden into the future.

In the beginning, it wasn't a garden at all. Instead, Ninfa was a town, one that has been referred to as the medieval Pompeii.

"Ninfa is a coincidence of history, botany, and art," says Lauro Marchetti, the garden's curator. "The first inhabitants were the Romans, who thought it so beautiful they named it after the nymphs they believed resided here."

By medieval times, Ninfa, about 44 miles from Rome, had grown into a thriving town with 2,000 inhabitants. In 1297 Pope Boniface VIII - the patriarch of the Caetani family - gave Ninfa to his nephew Pietro. Eighty years later there was a civil war in the church, with the Caetani family on the losing side. As an act of revenge, the victorious pope, Clement VII, destroyed the town.

Ninfa lay abandoned for six centuries - its churches, castles, and houses open to the elements. Nobody wanted to resettle the area - it was difficult to defend and malarial mosquitoes had colonized nearby marshes.

Then, in 1920, Onorato Caetani married English aristocrat Ada Wilbraham. As soon as she set eyes on Ninfa she made it her life's mission to restore it.

An artist, Ada would paint magnificent visions of the ruined castles and palazzi of Ninfa surrounded by plants and in harmony with the natural environment. She showed her canvases to the gardeners, who would try and re-create the pictures in nature.

When Ada died in 1934, her second son, Prince Roffredo, and his Boston-born wife, Maurgerite Chapin, continued to try to bring to life his mother's visions for the garden.

As Princess Maurgerite Caetani, she became a leading literary patron. At Ninfa, she added numerous shrubs to the trees that her mother-in-law had planted in the garden to form a foundation or backdrop for the landscape.

Today's garden is mostly the vision of Maurgerite's daughter, Lelia. Mr. Marchetti first met her when he was 7 years old. "I was born on the farm next to Ninfa," he explains. "One day I was playing here, and when they saw me running around in the garden, looking at the birds and animals, they 'adopted' me."

Lelia was the last in the Caetani line - she had no heirs to carry on her work. "One day she asked my father if I could come and live here as an apprentice. He agreed. When I asked Lelia how long should I come for she smiled at me and said, 'the rest of your life.' "

Before Lelia died in 1977, she created two foundations to ensure the garden's survival. As planned, Marchetti took over stewardship of the 200 acres of gardens and buildings.

"I have to think of the garden as a picture, and give maximum freedom and expression to the plants, animals, and birds," he says of his gardening philosophy. "Lelia never chose a plant for botanical reasons, only for artistic ones."

Everywhere at Ninfa there is evidence of this philosophy at work. A majestic cypress tree, for instance, reaches for the sky, with a rose piggybacking along its trunk. In a botanical garden, the rose would probably have been uprooted as soon as it tried to share the same space as the tree, but at Ninfa it is what Marchetti calls a "happy marriage."

The landscape is dotted with archaeological treasures - a bridge and seven churches from Roman times; a castle, a town hall, and city walls from medieval times, all in a state of arrested decay. Lush plants wind up and over most of them - from wisteria dancing in the wind on a bridge to hedges of lavender and rosemary lining pathways.

One of the most striking sights in the gardens is the church of Santa Maria Maggiore. Barely standing, one of its walls still bears a Byzantine fresco that glints in the dappled light of the sun as it passes though the shade of cedar trees.

Ninfa was first opened to the public by Lelia Caetani, who wanted to share the garden with others. Marchetti has taken his mentor's approach to sharing the gardens. He believes that Ninfa heightens the environmental consciousness of its visitors: "I remember groups of schoolchildren visiting at a time when there were three very rare black tulips flowering here. The first one was taken, then the second - I was desperate. So I put a sign by the third tulip saying, 'I am the last of three brothers. If you pick me, the others won't be able to live in the future.' After that, every day I found children here guarding the third tulip, which lasted for a month."

Sheltered by the Lepini Mountains and with the nearby Mediterranean Sea providing mild temperatures, Ninfa is blessed with a climate that allows tropical plants - such as orange and banana trees - to flourish. The walls also provide a sheltered microclimate for tender plants. The flowers growing here come from all over the world.

Marchetti lives in the only building the Caetanis restored at Ninfa - the medieval town hall. During Maurgerite Caetani's time, this was a literary retreat. The eccentric princess would host writers such as Ezra Pound, Henry James, and T.S. Elliott.

Her paintings hang on the wall of Marchetti's home. "When I wake up in the morning I can look at her visions of Ninfa, and then open the window and see how I am doing," he says.

Marchetti is expanding Ninfa. The adjoining farm where he was born is now part of the garden. A giant lake and corridors of walnut trees have been added. In common with all gardeners, he puts his faith in the future. "Like Lelia, I'm looking out for a person to take over from me in the future," he says. "But it would be best if that person was a musician or an artist. Ninfa is not about botany. This is not a botanical garden, but a spiritual garden."

Some would say it's an enchanted garden, the stuff of dreams. But Ninfa is real - the blending of the 20th century work of the Caetanis and Marchetti and a medieval town that didn't quite disappear.

• Ninfa is open from April to November on the first Saturday and Sunday of the month. The foundation limits the number of visitors on any given day. For tickets and information, call Fondazione Caetani , 011 39 06 68803231, or the World Wide Fund for Nature, 011 39 06 84497206.