Llanelli is doubling as Switzerland for a new film of the children's Alpine classic Heidi starring Diana Rigg.
Former Avenger Diana Rigg heads the cast of the new film
The former Bond girl and Avenger is heading the cast retelling the story of the orphaned Swiss girl first published in book form 120 years ago.
Stradey Castle is being used for interior shots although outside scenes will be captured back in the Alps.
Llanelli beat off competition from Cornwall as Wales-based production company CF1 searched for locations.
Company managing director David Ball said: "I was able to convince the producers that Wales was not only a better financial bet, but also preferable in visual terms.
"They are extremely happy with our choice of Stradey Castle and, once again, we are able to boost the local economy with our expenditure in the region.
"In a nutshell, the film wins and Wales wins."
Given its Alpine setting, the exterior scenes for Heidi will be shot in the real Alps.
Stradey Castle is being used for interiors until mid December.
Rigg is best known for playing Emma Peel in the 1960s cult television series The Avengers.
She is joined in the cast by Max Von Sydow, Geraldine Chaplin and Emma Bolger as the title character.
Von Sydow , who has a long list of film credits that includes The Exorcist, Flash Gordon and the Tom Cruise film Minority Report, is playing Heidi's grandfather.
The production was brought to Llanelli with the help of the Wales Screen Commission and Carmarthenshire Council's tourism department.
Matthew Parry, a film liaison officer with the commission, said Wales was increasingly being used to double for other countries on the silver screen.
"Wales has a great history of doubling for all sorts of locations.
Filming is taking place at Stradey Castle until mid December
"From Russia with Love, Tomb Raider with north Wales doubling for Cambodia and of course the last Bond as well where we were doubling for Vietnam and Korea."
The film is set for an international release next year.
The editor of Llanelli's weekly newspaper, the Llanelli Star, Andy Pearson said people in the town were excited by the filming.
"It's fantastic. For so many years the secret has been dormant that the area is indeed one of the best back drops you could possibly have.
"Stradey Castle is a lovely old mansion with castle like rooftops, it's got towers, it's got turrets, it's got lovely big luxurious bay windows, tall chimney stacks and everything like that.
"We are going to go silver screen mad - so good for Llanelli."