They don’t come any more fairy-tale than this: with soaring stone walls, towers, battlements, dungeons and tales of buried treasure, Il Castello di Montegualandro, on the Tuscan-Umbrian border, is straight out of a picture book.

Perched on a hilltop above Lake Trasimeno like “a falcon surveying its prey”, as one poet described it, and surrounded by 32 acres of pine forest and olive groves, the site was once owned by Charlemagne. So its owner, Claudio Marti, 69, a former Alitalia pilot, cuts a rather prosaic figure in his short-sleeved shirt and sunglasses.

Marti, however, deals in his own brand of chivalry: he and his wife, Franca, 65, rescued the castle from its near derelict state after buying it in 1985, rebuilding and restoring, with original materials where possible, to create a unique 28-room home.

“I was approaching retirement and looking for a country house in the area – but not a castle!” says Marti, who was living in a Roman palazzo apartment at the time. “We looked around for two years and couldn’t find anything interesting.” When they saw the castle, though, they knew this was it. “The place was so fascinating,” he says. “We didn’t start the actual renovation until 1987 – first, we needed to research the history.”

Montegualandro’s story is a long and bloody one: the castle and its outbuildings, rebuilt again and again over the centuries, passed from the hands of the Holy Roman Emperor in AD802 into those of various lords. Used as a garrison for papal troops thanks to its strategic border position, it ended up, in the 17th century, in the possession of the noble Ranieri family, who, in the 1950s, partially restored the curtain wall, punctuated by several towers, and the walls of the main castle, but later abandoned it.

“It was practically a ruin when we found it,” Marti says. “In fact, over the years, the castle has been destroyed five or six times, so there was a lot of work to do. There was no road up to it, no water or electricity; most of the roofs were down, and there was no glass in the windows.” Not surprisingly, he says, they got it for a good price: “Not many people were looking for something like this.”

It was only in 1992 that the Martis could move in, after carrying out an immaculate refurbishment. They used 13th-century plans to keep the property authentic. “We didn’t want to change a thing,” says Marti, whose work was closely overseen by the ministry of culture, as the property is classified as an ancient monument.

Indeed, entering Montegualandro (up a steeply sloping, deeply grooved stone ramp – ladies, leave your Prada heels at home), it is hard to believe you are a few minutes from the autostrada and just five miles from the stylish Renaissance town of Cortona. The main castle, dominated by its tower, and with a range of outbuildings lying inside the protective circular shape of the castle walls, is like an ancient village in miniature.

In the ground-floor hall of the main castle building, centuries of history are laid before you, with gothic stone arches and walls of Renaissance brickwork soaring above. Living space here comprises a two-bedroom keeper’s flat, a huge, vaulted reception area and a workshop.

Up the winding stone staircase to the first floor, you find the large, homely kitchen/dining room; another floor up are a living room, with a balcony providing panoramic views of the lake, and two more bedrooms. From here, you can access the tower, with its wood-panelled library.

There are four further staff bedrooms in the main castle, plus the one unrenovated area of the building – a huge mansard space, still to be plastered and painted, but with planning permission in place to create a four-bedroom, two-bathroom flat.

As the couple spent about £2m on the restoration project, they made sure they maximised Montegualandro’s earning potential. Next door to the castle lies a low, red-roofed building, dating from the 16th century, in which they have created five holiday flats, restored in traditional Tuscan rustic style with terracotta floors. They can be rented out all year round for £550-£630 a week.

Here, too, the Martis have sought to conserve the castle’s original character: one flat contains the castle kiln, where pottery and bricks were produced; another has the oven. The couple have also supplemented their income by recultivating the surrounding olive groves – the estate produces about 2,000 litres of organic olive oil every year.

Then there is St Nicholas’s Church, built in 1230, which is used for weddings. Above the altarpiece hangs a reproduction of The Virgin and Child with Saints (Nicholas being one of them), a 15th-century painting by Lucca Signorelli, one of Cortona’s most revered sons.

Despite the castle’s potential as a venue for upmarket events, the Martis are selling up, to be nearer their daughter in Siena. “We want another historic home,” Marti says, “but, after all this work, we need a rest.” The castle may be unique, but with a guide price of about £11m, prospective buyers aren’t exactly storming the walls.

“The castle would suit someone who wants a very unusual and historical home. And the buyer would have to be that rare combination – highly cultured, but very rich,” says Roger Coombes, director of Cluttons Italy estate agency, based in Perugia, which is selling the property. “As always in Italy,” he adds, “the price is negotiable.”

The price is not the only drawback. The new owner will not get the title of Count or Countess of Monte Gualandro, which once came with the property – the honorific is no longer recognised in Italy – and will have to open the place one morning a month to enjoy the tax benefits associated with living on such a historic site.

On the other hand, if you were looking to fly in your friends for some glamorous entertaining, Perugia airport is only 30 minutes away, and the castle has its own helicopter-landing field. If you felt antisocial at the last minute, you could pour boiling oil over your guests as they arrived – through the special holes in the ramparts.

Furthermore, you might just strike it lucky. During restoration work in the 1950s, a wall inscription dating from 1521 was found, indicating that a huge stash of gold and silver bars was hidden somewhere in its grounds. It has yet to be discovered.

Il Castello di Montegualandro is for sale for €15m with Cluttons Italy; 00 39 075 845 0100, www.cluttonsitaly.com

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