The success of Downton Abbey is a tribute not just to Julian Fellowes' ability as a dramatist, but to the enduring popularity of his subject: the country house. Highclere Castle, playing Downton Abbey, is the real star of the show. The British are in love with the "big house", the centre of a landed estate, and for centuries the dynamic heart of whole societies and economies in rural Britain.

The wealth and status of British landowners over the centuries and the impact of primogeniture in keeping estates intact has meant that Britain has a wealth of these magnificent properties, from mellow mansions nestling in the English shires to the palaces erected by Scottish dukes and earls in the border counties, and every year millions of us visit them.

The grandeur of these mansions - the succession of gracious rooms with their elaborate decors and furniture, pictures and silver acquired over centuries, together with the gardens, complete with orangeries, vineries and summer houses, which lead us, via walls or ha-has, to spacious grounds and ornamental lakes and beyond to a landscape sculptured for its vista with, perhaps, eye-catchers and follies in the shape of temples, hermitages or bogus ruined castles - is the most obvious attraction. These estates inspire mixed feelings of admiration, envy and imagined proprietorship, and perhaps a certain pride that the British aristocracy and gentry had, along with privilege and wealth, taste and aesthetic sensibility.

But the architecture, the interior design, the art treasures, the gardens and the cultivated surrounding landscapes are only part of the appeal. Many visitors, of course, are well informed about architectural history, or are serious gardeners, and if not all visitors know Jacobean from Victorian Gothic, appreciate a Joshua Reynolds or a Claude Lorrain, or know something of the way gardens and surrounding landscapes were designed and redesigned over time, a large proportion do.

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By the 18th century, the habit of visiting other people's houses had become established in this country, and those with the dress and deportment of gentlefolk were readily admitted - as Elizabeth Bennet found when Mr Darcy's housekeeper proudly showed her around Pemberley. Today, many houses like Pemberley, whether still in private hands or belonging to the National Trust or English Heritage, are accessible to the paying public and we are as taken with them as was Jane Austen's heroine. One reason why we are so intrigued by the great house may be that we are a nation of house owners or house owners manque - for if there are more great houses in Britain than in any other country, more Britons own their own modest houses than is the case elsewhere in Europe.

The semi-detached or terraced house may be far from a mansion, but its owner lavishes care and money on improvements: an extension, a conservatory or a "makeover" of the garden. Television producers have pounced on this obsession. Few would have the perseverance or skill to renovate a decayed manor house, but this too is the subject of a popular programme. It may be a great leap from the new patio or the introduction of a birdbath to the cascade at Chatsworth or the Duchess of Northumberland's new garden at Alnwick, but how wonderful to see what can be done on a grand scale!

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Perhaps the most potent attraction, for many visitors, is a fascination with a past way of life. In recognition of this, many houses now stage enactments and have staff dressed up in period costumes, while at Hampton Court there are "live interpreters". The appeal of great houses and manor houses is inseparable from a curiosity about the history of the people who lived in them. By far the most popular houses are either still owned by the families who lived there in their heyday or still bear the imprint of the families who once lived there and whose portraits, photographs and treasures still linger. A glimpse of the often shabbily tweeded aristocratic owner is the high point of many visits.

But, of course, the smooth running of the house depended on a large army of servants. In addition to the indoor servants, the maintenance of the household and the leisure pursuits of the family required coachmen and grooms, gardeners and gamekeepers, while the home farm and the dairy provided its sustenance. For today's visitors, the service and domestic side of a house can command as much attention as the formal rooms. Magnificence and art treasures demand admiration, but often it is the everyday artefacts of the past, the equipment of kitchens or the cots and toys in nurseries, or even a room with an array of chamber pots, that most intrigue. It is the relationship between two parallel but closely connected worlds - that of the upper orders, who lived in those splendid rooms, and that of those who cooked their meals, waited upon them, dressed them, carried coal and logs to innumerable fires, drove the carriages and made sure there was plenty of game to shoot - that interests many. How did the two worlds regard each other: with disdain and resentment or with paternalism and loyalty? It is this subject and these questions that have made Downton Abbey such a popular and gripping series.

For the aristocracy and gentry, servants were both a necessity and a problem. Their position and comfort depended upon them, but as privacy became more important, it was difficult to escape from their watchful eyes. The origins of the country house lay in fortified dwellings, whether castles, tower houses or manors, where retainers were essential, not just as servants but as armed supporters. In such an ambience, the privacy of lord and family were of minor importance. Superiors and inferiors ate together in the great hall, with distinctions preserved, as in the colleges of older universities today, by the division between those on the lower tables and those on the high table on its dais, and if the lord and family could retreat to the solar, life was essentially communal. As defence became less important and comfort and gracious living more desirable, the servant problem became more pressing.

By the 18th century, privacy had become of great importance. Villages close to big houses were demolished and new "model" villages were built away from these estates, thus improving the view and keeping workers at a distance. Inside the houses, such distance between the orders was impossible, but a partial answer was to divide the lower floors of the house by the "green baize door", which demarcated the family's rooms from the service area - a horizontal divide, as opposed to the vertical one of the "upstairs, downstairs" of the households of the wealthy urban upper-middle class, and one maintained by separate staircases to the upper floors. A more obvious and sophisticated demarcation of separate spheres can be found at Cragside, the house built in 1863 for the great industrialist and gun-maker Lord Armstrong, where alternative routes around the house are denoted by doors with higher or lower arches, the former for family and guests, the latter for servants. The barriers were, nevertheless, permeable and, at mealtimes especially, conversation was inevitably overheard by servants waiting at table, even if the 19th-century introduction of the "dumb waiter" made for privacy at breakfast.

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On the service side of the green baize door was a complex society with a strict hierarchy, but one that had many advantages: there was plenty of company, food was plentiful, the owners and their families were often absent in London for the Season or at their shooting lodges, and there was a career ladder. To many modern minds, the deference and acceptance of "one's place" that was inseparable from this life may appear insufferable, but much of the poor reputation of service comes not from the great house but from the fate of the maid-of-all-work in a lower-middle class or even artisan household, where the vast majority of the million and a half servants employed in 1901 worked. This was a very different and much inferior experience: no other servants, no opportunities for promotion, food that was often exiguous, and demeaning rules such as "no followers" (boyfriends).

Servants in the country house often retained their positions for many years, perhaps rising in the hierarchy over time, and their children frequently followed them into service with the same family, so relations with their employers rarely remained impersonal and, despite the green baize door, an older notion of family that included servants and retainers lingered into the 20th century, making for paternalist and even affectionate relations. Servants were rarely dismissed and even then, usually, with reluctance. Butlers, the rulers of the servant world, were notoriously inclined - perhaps because of their control of the wine cellar - to take to the bottle, and many families tolerated such characters (although the habitually inebriated butler of Mrs Ronald Greville of Polesden Lacey, who upon receiving a note from his employer, "You are very drunk. Leave the room at once", staggered around the table and delivered it to an astonished Sir Austen Chamberlain, was an unusually spectacular example). Carson, Downton Abbey's butler, is by contrast a model of propriety.

The mistress of the household, on whose shoulders lay the responsibility for the smooth running of the household, was, of necessity, in a close relationship with her servants. She and her personal maid often became friends and confidantes, and many a lady was bereft when her maid or housekeeper left to get married. Such a close relationship is depicted in Downton between Lady Grantham and her rather sinister maid, O'Brien.

Close proximity had its dangers, however, and as Lawrence Stone has demonstrated in his work on the history of divorce, the witnesses in cases where a married woman was accused of adultery or, in that wonderful phrase, "criminal conversation" were invariably disgruntled domestic servants - "What the butler saw" could be very inconvenient. When, in Downton, Lady Mary is unfortunate in having the "Turk in her bed" die on her, it is the servants who know the secret. Ladies of the house would occasionally fall for footmen who were, after all, chosen for their fine legs, while husbands and sons were not impervious to the charms of pert maids, as is the case with Lord Grantham. Coachmen may have tucked ladies into blankets or furs, but at least they sat in the box, unlike the newfangled chauffeurs, who (like the Irishman who drives Lady Sybil) had greater opportunities for intimacy.

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Downton Abbey begins in the last great period of the country house before the First World War, a final blaze of glory before a decline, which, speeded by taxation and both world wars, continued until the 1950s, and it has an elegiac and nostalgic quality. Yet the decline of the bricks or stones and mortar has been halted. The magnificent town houses of the aristocracy have all gone, and hotels or blocks of luxury flats have taken their place, but country houses are conserved and cared for and more remain in private hands than seemed possible when James Lees-Milne visited owners, living in decayed and uncomfortable grandeur in the post-war years, offering them salvation at the price of transferring future ownership to the National Trust. A surprising number remain in the hands of families that have lived in them for centuries, while, as in the past, new wealth from commerce, industry or the Empire bought status, respectability and comfort in a landed estate - so hedge-fund founders, film stars and erstwhile-rebellious popular musicians invest in the ultimate British status symbol, the country house.

Britain's great houses have a central place in the historical memory of the British and one that is constantly refreshed as, every year, more than one in five of us pays a visit to one of them. Not everyone approves, of course. Some find our obsession with "heritage" unhealthy and see the country house as a symbol of oppression, from the "Norman Yoke" to the high tide of Edwardian plutocracy. Why, they ask, can't the British be forward-looking and modern? Why do they still "dearly love a lord" and why would they rather sit next to a countess at dinner than a sociologist? Why, above all, do they insist on spending so much of their weekends and holidays imbibing the atmosphere of a past way of life? Why does the National Trust have far more members than all the political parties combined? Most of us, nevertheless, are happy enough visiting the splendour that only great wealth could create and making an imaginative entry into a very different way of life. That's why 8.1 million of us tuned in to watch Downton Abbey on Christmas Day.

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