It's all been done in the nick of time," says Patrick Eyres, at Wentworth Castle, outside Barnsley.

Patrick, his fellow trustees and the staff here are justly proud of what they have achieved over the last five years. For by the late 1990s, South Yorkshire's only Grade I listed landscape was in a parlous state.

"The 18th- and 19th-century plantings were in direct competition," explains Patrick. Experts from Kew Gardens estimated that the gardens were only five years away from being lost forever. And some of the garden's buildings were in even worse shape. Structural engineers brought in to survey the parkland's rotunda, for example, told the trust that it might collapse the following month.

Wentworth Castle's annus mirabilis was 2003. First the trust was awarded £10.3m by the Heritage Lottery Fund. Then the Victorian conservatory and Stainborough Castle – a mock ruin on the hill above the house – came third in the BBC's first series of Restoration.

A first phase of works took place in 2005-6, and Patrick shudders to think what might have been. "If we hadn't re-roofed the house and rebuilt the home farm when we did, they would have been devastated by last summer's rain." It's an epic project, and a new beginning for a unique landscape. But to understand just how exciting Wentworth Castle is, Patrick says you have to know something of its maker – Thomas Wentworth, Earl of Strafford (second creation).

Not everyone found him attractive. A contemporary described him late in life as "a loquacious, rich, illiterate, cold, tedious, constant haranguer in the House of Lords, who spoke neither sense nor English". But he was also, Patrick emphasises, an 18th-century man of culture, a famous soldier, and a statesman and peacemaker of European importance. Thomas Wentworth had expected to inherit the neighbouring Wentworth Woodhouse estate, which had been rebuilt by his ancestor, the first Earl of Strafford – "Black Tom Tyrant". He was a henchman of Charles I, who became the fall-guy in the King's power-struggle with Parliament and was executed in 1641. The Wentworth Woodhouse estate passed to Black Tom's son and when he died in 1695, it went to Strafford's cousin, Thomas Watson. Thomas Wentworth was furious, believing he had been unjustly disinherited. He commenced a life-long campaign to upstage his cousin. This included reactivating the title Earl of Strafford (hence the term "second creation") and secretly buying the Stainborough Hall estate, six miles away from Wentworth Woodhouse. By 1717, however, his usurping cousin was not the only thing on Thomas Wentworth's mind. He had fallen foul of the new Protestant king, George I, and the ruling Whig party, and was forced into political exile.

Back in Yorkshire, at Stainborough, he directed a significant part of his vast energies into supervising the creation of a magnificent house and garden. It was intended, on the one hand, to celebrate his own military and political achievements. But it was also meant to surpass Wentworth Woodhouse. By the early 1730s, he was ready to rename Stainborough Hall as Wentworth Castle.

Wentworth Castle also contains clues to another, shadowy existence. By the 1720s, Strafford had become the northern leader of the Jacobites. Their plan was to overthrow George I and restore the Stuarts in the shape of the exiled Catholic, James II, his son, and his grandson, Bonnie Prince Charlie.

"Wentworth Castle is one of the most exciting landscapes in the country, because treason is everywhere to be found," says Patrick Eyres. You discover it in the carved oak leaves which embellish the magnificent interior. They were the symbol of the Jacobites after Charles II escaped the Roundheads by hiding in an oak tree. The monuments here, and their inscriptions, even the shape of the "wilderness" gardens, can all be read as covert celebrations of Strafford's dangerous politics.

"Most other aristocratic families would have obliterated all this," says Patrick. But Strafford's son, William, simply moved on, leaving the references to his father's dubious politics intact. William made his own huge contributions to the landscape we see today, building a new Palladian front, landscaping the park in the style of Capability Brown, and adding further monuments and garden features.

William died childless in 1791 and the estate passed to the Vernon-Wentworths who were to enjoy it for another century and a half. Wentworth Castle shared the fate of many country houses in the early 20th century. Its upkeep became too much for its owners; it was neglected and then requisitioned by the army during the Second World War. In 1948, it was put on the market and bought by Barnsley Council who turned it first into a teacher training college, and later into the Northern College for Residential Education. Money was tight, and by the late 1990s, the situation was becoming critical. The turning point came with the creation of the trust and the reunification of the garden and the park. The crucial Lottery funding was followed by the publicity and goodwill generated by the Restoration programme.

Five years on, the 18th-century "wilderness" garden and the 19th-century rhododendron collection are thriving; Stainborough Castle has been repaired and many of the garden buildings have been restored and refurbished. Most recently, the trust have opened an adventure playground. Wentworth Castle must now count as one of Yorkshire's best family days out. But there is more to do. Patrick was here to discuss the next phase with the castle's new heritage director, Steve Blackbourn, and Jane Furse, another long-time trustee. In June, the trust heard that their latest Lottery bid had not been successful. They had hoped, finally, to restore the Victorian conservatory, and to begin work on the kitchen garden, whose enigmatic ruined walls dominate the entrance to the site. The projects have now been separated and Steve will go back to the Lottery later this year with a new bid for the conservatory. "It is vital we address the conservatory quickly," he says. "It cannot survive much longer."

Jane agrees and emphasises the building's importance. "It's not that it's early, or that it's metal, it's the fact that it incorporated from the first what was at the time cutting-edge technology – electricity. We are at risk of losing what we know of early electrical technology. Often it has just been ripped out of old buildings without thinking." The second aspect to the bid, the restoration of the kitchen garden, however, will have to be put on hold. But the trust are itching to get on with it. So, we put on our wellies and set out for a walk.

Disappointment follows. All that remains, it seems, is a bleak red-brick enclosure, overrun with rosebay willow herb, and housing the mass of timber salvaged from the restoration of the gardens. Surveying the desolation it is almost impossible to imagine the gardens as they once were – busy, ordered and fantastically productive. The kitchen garden of a great house like Wentworth Castle provided fruit and vegetables for the entire establishment all the year round. But all is not lost. The trust has been able to study the crucial correspondence between Thomas Wentworth and his head gardener, John Arnold. From this it is clear that Arnold and his staff were cultivating an almost unbelievable range of fruits – peaches, nectarines, apricots, melons, grapes, oranges and later the prestigious pineapple. And the list of vegetables he was providing for Wentworth's table includes now rare delicacies such as salsify, scorzonera and the asparagus pea.

They were using every contemporary method possible to extend the growing season at either end. This was especially necessary because the site was high and exposed. Unsurprisingly John Arnold (who had previously worked near London) spent a lot of time complaining about the weather.

Some of Arnold's 18th-century methods are still in use, such as "forcing" (where vegetables are grown in heat and darkness in order to produce early and especially succulent shoots). Others are now period-pieces, such as hot beds (where warm fermenting horse manure was covered with topsoil, planted-up and then topped with a glass frame).

The most ingenious and fiddly methods, however, were reserved for fruit. Here the garden's walls came into their own. Warm south-facing walls were used for the choicest fruits, while shadier and cooler north-facing walls supported sturdier varieties. Trees were painstakingly trained to grow right up against the wall – in espaliers, cordons or fans – to make the most of the heat absorbed and released by the bricks.

But Jane is moving us on to examine a particularly neglected corner of the garden. Here, the brick walls are crumbling away to reveal a system of air spaces and flues. Fires were lit at the base of these walls and hot air funnelled through them. Heated walls like this were used to protect and ripen one of Thomas Wentworth's most prized crops – sweet grapes.

We push on, through brambles and stinging nettles, to the final structure Jane and Patrick want to show me. It doesn't look like much – a boarded-up patchwork of stone and brick with sheets of blue plastic protecting it from the elements. But it is, in fact, a ruined 18th-century orangery.

Like the conservatory, Jane tells me, the orangery would have been heated (by means of pipes under the floor), and like a conservatory it was used to protect delicate and prestigious crops, in this case citrus trees. The trees spent the winter inside and then would have been moved out into what Jane is convinced was a sunken garden. The empty orangery would then have been used as an outdoor dining room.

The presence of the orangery and its garden show how important the kitchen garden was to Wentworth Castle's 18th-century owners. We tend to think of kitchen gardens in 19th-century terms – as utilitarian, almost industrial spaces. But when this garden was built its owners were so proud of it they wanted to eat there. In fact, Thomas Wentworth was so proud of his produce he had his coat of arms, coronet, star and garter tattooed onto his melons, so that the design grew as they did.

So what next for the kitchen garden? Steve plans to begin raising funds early next year for an "enabling project" which will involve bringing in landscape archaeologists to excavate the gardens and then some basic landscaping.

In the long-term the trust would like the garden to be used by children, students and adults to learn about growing and cooking, heritage varieties and seasonal eating. "From plot to plate" as Steve puts it, and a fitting rebirth for a once beautiful and useful place.

The gardens and park at Wentworth Castle are open daily, 10am to 5pm. Guided tours of the house take place
on Sundays.