As you would expect of a 600-year-old castle with links to every French historical figure from Cardinal Richelieu to Napoleon, the Château de Pierrefonds is steeped in legends. Its vast stone walls and corridors are filled with statues of fabulous creatures, from giant lizards to gryphons. Its giddy towers and turrets are reputedly home to 14 ghosts.

On this sweltering summer day, however, the collection of characters gathered in the grounds must be the most fantastical this edifice has seen. Near the drawbridge a medieval knight is doing battle with a creature that is half-Davros, half-Darth Maul. Deep in the dungeons a BBC camera crew is filming a ferocious talking dragon that sounds suspiciously like John Hurt.

The BBC has come to Pierrefonds, an hour north of Paris, to film a new version of one of the most enduring of British legends, Merlin. The French castle’s fairytale appearance was deemed superior to Arundel, Alnwick and other heritage-laden English castles as the latest incarnation of Camelot. The producers hope it will help to pull off a very modern alchemy: turning a Saturday night drama into ratings gold.

The idea was born two years ago when the producers Johnny Capps and Julian Murphy, from the independent production company Shine, were working on the teen-horror hit Hex for Sky. “We had enjoyed doing a high-concept show and it cemented our feeling that we wanted to do more,” says Capps, hiding from the sun in the camped village that the BBC has erected to accommodate its huge crew. “We started looking at different worlds and mythologies. We spotted that no one had attempted Merlin for a while.”

The success of Doctor Who, and to a lesser extent Robin Hood, on Saturday nights encouraged the pair to develop an ambitious new version of the legend, geared for a new type of audience. “We didn’t want to make a 15th-century version of King Arthur and the Knights of the Round Table. We wanted to produce something that merged the legend with big family entertainment,” Murphy says. “That’s something Doctor Who has made possible. The BBC is more willing to commission innovative and fantastical programmes.”

As they looked at how to approach Merlin, their first influence was the US show Smallville. Merlin aims to do for the Arthurian legend what Smallville has done so effectively for the story of Clark Kent – recounting his life long before he became Superman. Set in a Camelot that existed before the golden age, it is a kingdom in which knights sit around a square table, where Lady Guinevere is a servant girl called Gwen, where magic is banned, and Merlin and Arthur are young men.

“Merlin is a boy who works as Arthur’s manservant and has to hide his abilities,” Capps says. “Arthur is a young man, rather like Prince Harry, who likes to party, enjoys his status, but occasionally shows kingly qualities. That then gave us great possibilities for taking the story on. We could follow the two of them to see how Arthur unites the land of Albion and Merlin becomes the great wizard.”

Happily, Julie Gardner, the executive producer with Russell T. Davies of Doctor Who, was looking for a drama to fill the void that will be left when the Doctor takes a break over the next year or so. She says that she wanted a story in which protagonists regularly dealt with life-or-death choices. “You also need characters with whom the audience can empathise. That doesn’t mean characters who are nice all the time but characters whose trials and tribulations you want to follow.”

Gardner likes her casts to blend the familiar and the unfamiliar. Relative newcomers include Colin Morgan as Merlin, Bradley James as Arthur, Katie McGrath as Morgana and Angel Coulby as Gwen. The veterans include Anthony Head as the autocratic king of Camelot, Uther Pendragon, Richard Wilson as Gaius, the physician who becomes Merlin’s mentor, plus Hurt lugubriously voicing the Great Dragon.

During a break in filming, Morgan, who was noticed playing the lead in a stage adaptation of the novel Vernon God Little, says: “The story is so fantastical. We are not saying this is the truth. What we are saying is that this is a treatment of history that has not been written or seen before.”

Merlin will have its share of special effects, from gryphons to flying witches. But it’s not about horror. Gardner says: “If I’ve learnt anything from working with Russell, it is that fun should be at the heart of it. It needs to be colourful and noisy and bright because drama on Saturday night is competing with The X Factor and other big, blowsy reality shows.”

To judge by the first episode, in which Merlin arrives in Camelot, the series certainly has its big and noisy moments. Its high-definition look was deemed impressive enough to justify a public premiere at the IMAX cinema in London. The central performances augur well, and the series has already been licensed to NBC for US broadcast. It’s almost unheard of for a big American broadcaster to buy an untested show sight unseen.

Yet the makers know that long-term success is by no means assured. “We are in a very difficult spot. We will have a very large magnifying glass put on us,” Capps says. “Some people who love the legend of Merlin will be horrified when they see that he and Arthur are contemporaries.”

There is talk of at least four series. “We have big plans for Merlin but you never know,” Capps says. “There is huge expectation and if it touches people’s hearts then it will be a success. But it will in the end be in the hands of the audience.”

Merlin, Sat, BBC One, 7.30pm

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