ON the narrow Devon lane from Chagford to Drewsteington, the road suddenly dips to reveal, high above, the stunning sight of a castle. Not the standard Norman pile - this is no fallen-down ruin but a real, inhabited castle (the sight of a TV aerial on the roof confirms it).

Even from a distance, you can tell there is something not quite genuine about the building. It looks rather too neat - and there is a curious mixture of styles: part Norman, part Tudor and entirely fantastic. Castle Drogo looks new because it is. Britain's only 20th-century castle, it was begun in 1910, finished in 1930.

We started this Swot Spot series down a Welsh coal mine, learning about the hardships endured by the workers whose sweated labour helped to create the modern prosperity of Britain. It is fitting, therefore, to end the series by learning about those who benefited handsomely from this new wealth.

Castle Drogo, now owned by the National Trust, provides a glorious insight into a unique slice of life. The place is a striking monument to British entrepreneurial nous. When you take that first glimpse, what you see is a fortress built out of tea.

You may still see the name Home and Colonial Stores on old shop fronts that have managed to resist the unkind attentions of the high street developer, conjuring pre-Second World War Britain (Betjeman celebrates it in his poem 'Myfanwy': 'Home and Colonial, Star, International / Balancing bicycle leant on the verge'). It was one of the pioneering high street retailers that, along with Lipton's and Sainsbury's, succeeded in revolutionising the British grocery business at the end of the 19th century. Its founder was Julius Drewe, the son of a clergyman, born in 1856.

Julius was employed by his uncle to buy tea in the Far East, and proved surprisingly adept. He returned to England at the age of 22, saw an opportunity to sell cut- price tea direct from its country of origin to the public, opened his first shop in Liverpool in 1878, and in 1883 established his first London outlet in Edgware Road. By 1890 there were 106 Home and Colonial Stores promoting the sale of India tea - as a result of which it began to replace China tea in British affections.

By 1890, when he was still only 33, Julius Drewe had made enough money to retire and to set himself up as a country gent. With his new wife, Frances, he acquired Culverden, a mock castle in Kent, and nine years later the family moved into Wadhurst Hall in Sussex, purchased from a bankrupt Spanish banker.

But Drewe had ambitions to establish his family in a still more impressive country seat. Julius's brother had engaged the services of a genealogist who discovered that the Drewes could trace their roots to Devon, where they were originally related to a Norman baron - Drogo, or Dru - who had accompanied William the Conqueror. It was claimed that this Drogo de Teigne had given his name to the parish of Drewsteington.

Julius's cousin, Richard Peek, happened to be rector of Drewsteignton, and Julius knew the area. He quickly acquired 1,500 acres near Drewsteignton and employed the period's foremost architect, Sir Edwin Lutyens, to draw up plans for a genuine fortress to be built in the sort of commanding position where Baron Drogo might himself have built a castle.

Lutyens was given a budget of pounds 60,000 ( pounds 50,000 for the castle and pounds 10,000 for the garden), vast sums at the time, equivalent to around pounds 7m in today's money. Labour costs did not represent a significant part of that budget. The site manager was paid pounds 5 per week. Labourers, who could be sacked at an hour's notice, worked 50 1/2 hours a week (with a 30-minute lunch break) for 3p an hour.

If Castle Drogo can be described as a folly, it is truly a folie de grandeur. As well as the immense practical problems of the building work - constructing granite walls up to 12ft thick was a process not much tried since feudal times - the project was disrupted by the outbreak of the First World War. This not only removed two-thirds of the workforce, but in 1917 led to the death of Julius's eldest son, Adrian, who was to have inherited the castle, and who had been a guiding light in its planning.

Julius Drewe was able to live in the castle for just a few years before his death in 1931. Following the death of his second son, Basil, in 1974, the castle and 600 acres of land were given to the National Trust - the first 20th-century property to be acquired by the Trust.

To stroll through the interior is to enjoy an experience somewhere between Upstairs, Downstairs and Camelot. Lutyens achieved a miracle with the surprisingly intimate scale and design of the rooms. He designed much of the furniture, including what must have been one of the first fitted kitchens (since copied by a manufacturer, with each sale contributing a royalty to the National Trust).

If you want to show your children a very particular slice of early 20th-century life, Castle Drogo provides a vivid instant history lesson. When you look at the view of Dartmoor from the drawing-room, you will wish that the castle was yours and that you were monarch of all it surveys.

You would not, however, wish for the running costs. In its heyday it required 23 full-time staff. Now its annual heating bill is pounds 11,000, the electricity bill runs to pounds 9,000.

One thing children will certainly learn from their visit is that in those high days of capitalism the rich were indeed different from us. They were absolutely loaded.

Further information: Castle Drogo (0647 433306) is open every day except Fridays until 31 October, from 11am to 5.30pm. The gardens are open every day. Admission to the castle, garden and grounds costs pounds 4.60 (garden and grounds only, pounds 2). There is a licensed restaurant in the castle, a tea room in the grounds. The gardens have a croquet lawn for visitors' use. The castle is four miles south of the A30 Exeter to Okehampton road, and is reached via Crokernwell.