The manor of Mereworth came to Francis Fane, 1st Earl of Westmorland, in the early 17th century. Fane inherited it through his mother Mary Neville - who held the title of Baroness le Despenser in her own right - the sole daughter and heiress of Henry Nevill, 6th Baron Bergavenny. The fortified medieval manor stood on this ground, but by the early 18th century it was tired and unfashionable. John Fane, 7th Earl of Westmorland, decided to rebuild. He commissioned Colen Campbell - one of the leaders of the British Palladian revival, the man whose "Vitruvius Britannicus" published between 1715 and 1725 was effectively the manifesto for an architectural movement determined to reject Baroque excess in favour of harmonious classical proportion. Campbell's 1723 design was a direct homage. Palladio's Villa Rotonda, designed in 1567, was a square plan with four identical porticoed facades surrounding a central domed hall. Mereworth followed that scheme almost line for line.

The interior fitted out with talent imported from Italy. Giovanni Bagutti, a Swiss-Italian stuccoist who had worked on St Paul's Cathedral and Castle Howard, did the plasterwork. Francesco Sleter - a Venetian painter who became one of the most sought-after fresco artists in early Georgian England - painted the ceilings and walls. The dome rose above a central hall lined with classical figures, columns, and the kind of trompe-l'oeil architectural detail that made English country houses feel briefly like Italian palazzi. Outside, the landscaped park and valley contained surrounding pavilions and lodges, all also Grade I listed today. The Wateringbury Stream ran through the grounds and turned a fulling mill at the eastern end - the kind of small industrial operation that supported the estate's income alongside the agricultural land. In 1746 the Italian painters Francesco Zuccarelli and Antonio Visentini produced a "capriccio" - an imaginary architectural composition - featuring a view of the castle, confirming its arrival on the European cultural map.

The house passed through descent to the Barons Oranmore and Browne - an Anglo-Irish peerage whose third holder also became Baron Mereworth in 1926, so that since then the two titles have been united. The Oranmore family sold Mereworth in 1930. During the Second World War, the building served as a prisoner-of-war camp - one of dozens of country houses requisitioned by the British government for that purpose. In the 1950s and 1960s it was owned by the artist Michael Lambert Tree, who lived from 1921 to 1999. Tree was the son of Ronald Tree, a British MP and country-house host whose Ditchley Park became one of Winston Churchill's favourite wartime retreats; he was also a great-grandson of Marshall Field, the American department-store magnate whose Chicago fortune funded much of his branch of the family. His wife, Lady Anne Cavendish, was a daughter of the 10th Duke of Devonshire. Tree had inherited Mereworth from his uncle Peter Beatty, who died on 26 October 1949.

Since 1976, Mereworth Castle has belonged to Mahdi Al-Tajir, who paid $1.2 million for it. Al-Tajir was for a time the United Arab Emirates' ambassador to the United Kingdom; he also owns the Highland Spring bottled water company - the brand pulled from the Ochil Hills of Scotland, distributed across Britain and far beyond. He has been one of the wealthiest residents of the UK for decades. The house is not generally accessible to the public. Occasional guided tours have been arranged - rare windows into the dome, the painted ceilings, the surviving Italian stuccowork - but they are exceptions. Most Kentish people pass the gates without ever seeing inside. The pavilions are still there. The valley is still landscaped. The Wateringbury Stream still flows past the fulling mill. From the road, you catch a glimpse of the dome through the trees, white and improbable, and then it is gone.

Why does a near-copy of an Italian Renaissance villa exist in rural Kent? The short answer is the British 18th-century obsession with classical architecture - an obsession driven by the Grand Tour, by the Whig political class's wish to associate itself with the Roman Republic, and by a generation of architects led by Lord Burlington, Colen Campbell, and William Kent who genuinely believed Palladio had decoded the universal grammar of beauty. The longer answer is that John Fane could afford it, and that the Mereworth landscape - a gentle south-facing slope with views across the Weald - made the Palladian square-plan, four-portico template work surprisingly well. Climates differ. Palladio's villa stands open to Italian summers. Mereworth's porticos catch English rain. But the proportions still hold. The dome still rises in the right relationship to the four facades below. Standing in front of it, you can see exactly what Fane was buying: a fragment of Renaissance Italy, set down forever in the Garden of England.