Pierrefonds castle dominates the village of the same name in a forest northeast of Paris. “A big bugger,” says a fellow English visitor fluent in building vernacular. This is correct.

Pierrefonds is enormous, and famous in France as the crowning achievement of the architect Viollet-le-Duc.

Shortly, it will be equally famous in Britain as the star setting for the BBC’s upcoming series Merlin. Plundering Arthurian legend, it’s scheduled to burst out of the Saturday-evening Doctor Who slot: yet more historico-fantasy drama in the Narnia/Potter vein, it has Merlin and Arthur as young men involved in “magical adventures”. Whatever, it will certainly bring a degree of prominence to Pierrefonds. With towers, soaring walls and a masterful medieval presence, the castle stands in as Camelot throughout the 13-week series.

So, after 600 years of forbearance, Pierrefonds will, in addition to nobility, have celebrity. At the village cafe beneath the castle, they’re celebrating by charging £1.75 for an espresso. This is 40% more than I usually pay, even when I’m paying a lot.

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BBC location scouts scoured Britain and the Continent before happening on Pierrefonds. The chateau matched what is, apparently, the epic-scale fantasy of the show. “We wanted something that looked unbelievably photogenic and vast already,” says Merlin’s co-producer, Julian Murphy. “We didn’t want a main location where we would need to add CGI all the time.”

Viollet-le-Duc would have been thrilled. He was the chap who, from 1857, rebuilt Pierrefonds. Vastness and epic-scale fantasy were his stock in trade. Medieval legends were part of his vocabulary. For the bluffers among us, it’s perhaps worth recalling that old Viollet was the leading French architect of the 19th century.

He went around restoring France’s most notable medieval buildings, not necessarily to their original states, but as he thought they should have been: better, showier, cleverer. This was, and still is, terribly controversial in architectural circles - but it also explains why Notre Dame, Mont-St-Michel and Carcassonne look as they do today.

As it was one of his later projects, he really put the packet into Pierrefonds. Built in the 14th century, the castle was a ruin when Emperor Napoleon III decided he wanted it doing up - essentially as a smack-in-the-eye showcase for his own brilliance. Viollet-le-Duc got cracking, recreating the mighty Middle Ages elements with gusto, but drafting in elements fromother periods, notably the Renaissance.

The propaganda message was clear: France had always been a pretty terrific country; and, under Napoleon III, remained so. It was architecture as historical, and contemporary, spin.

Before we get bogged down here, let us move briskly on to the present, where the chateau still hangs over the village like one of those alien craft from Independence Day. It really is massively, quintessentially medieval, which was Viollet-le-Duc’s whole point. You could besiege it for a generation and still not get beyond the snack bar at the bottom.

Once up the hill and inside, you see more of what Viollet-le-Duc was about. The huge courtyard is trimmed with Renaissance galleries - mildly surprising behind outer walls from the Middle Ages. It has giant stone lizards as downspouts and, within the huge chambers, polychrome decor, as if several hundred primary-school kids had been let loose with stencil kits. Medieval folk liked their colour, but Viollet-le-Duc liked it better.

The chateau goes on. So could I, but it’s far more engrossing to see than to read about. Please just believe me when I tell you that it is 83% more interesting than most French castles.

The castle visit will happily fill a morning. You may then return to the neat little village below and, if you’ve been saving up, have a coffee. From there, take the forest byways north towards Réthondes. You will note a home-counties air, suited to a region that has been a retreat for the royal, rich and powerful for a millennium.

Once near Réthondes, follow signs for the Clairière de l’Armistice - the most significant forest clearing in Europe. It was here that the armistice ending the first world war was signed on November 11, 1918. Maréchal Foch and allied leaders arrived in one railway carriage, the German plenipotentiaries in another. The talks and the signing took place in Foch’s carriage.

The Germans got their revenge when Hitler forced the French to sign their submission in the same carriage in the same clearing on June 22, 1940. The Führer then had the carriage sent to Berlin, where it was burnt. So the one on show isn’t the real one, but a contemporary from the same series.

It’s flanked by a museum that, although fascinating, is surprisingly low-key for such an important spot. “We’re expecting Nicolas Sarkozy this November 11 - the president usually comes in years that end in eight,” says the curator, Capitaine Jean-Christophe Le Goaër. “If he can’t make it and sends his wife instead, we’ll not complain.”

And so, perhaps on day two, to the royal town of Compiègne - “royal” because French monarchs have been turning up here since they all wore skins. The key thing here, you will be told, is the Château de Compiègne, and it is indeed another monumental item. Triangular, too, which is odd. Chamber after salon is full of frothily sumptuous memories of Napoleons I and III, who were both regulars. In truth, though, the visit drags until you’re tempted to set about the guide with a Second Empire chair leg. You’ve seen Pierrefonds. I’d skip this one.

Wander instead to the River Oise, then to the main square, where the lavish townhall facade confirms that, though gothic masons knew when to start, they didn’t really know when to stop. Ambling in the town is a gentle delight. Once fully delighted, make time for the Mémorial de l’Internement et de la Déportation, two miles off centre on Avenue des Martyrs de la Liberté. This is a more sombre experience, a war-time transit camp for “undesirables” - Jews, communists, résistants - recently transformed into a quite exceptional contemporary museum.

With cultural duty done, you might like to cycle the forest paths. There is exhilarating potential for getting lost, which I exploited fully. You will be glad that this is the 21st century, so the forest is free of dragons, weird old wizards and hairy chaps with broadswords. In fact, the only living things are an odd deer - and a determined young lady pedalling fast towards me. With the entire forest of Compiègne to go at, she runs into my bike.

We both fall off. She is furious and really rather insulting. I react with Arthurian chivalry. She’s buried by the 750-year-old oak tree just outside the village of St-Jean-aux-Bois.

Travel details: VFB (01452 716842, www.vfbholidays.co.uk ) has three nights at the lovely, gastronomic Auberge à la Bonne Idée, in St-Jean-aux-Bois, in the Compiègne forest, from £307pp, half-board, in September, including Channel crossings. Or try Great Escapes (www.greatescapes.co.uk ) or Allez France (www.allezfrance.com ).

Merlin starts this Saturday on BBC1

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