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The makers of hit TV show Game of Thrones will be filming at a Welsh castle this week. 

American TV giant HBO that makes the show is spending four days this week setting up at Penrhyn Castle near Bangor. 

The National Trust, which owns the castle, is warning visitors that all photography will be banned within the historic building. 

The National Trust and film commission bosses have refused to reveal the nature of the production but the Penrhyn Castle website says the site will "star as a major film set, hosting the filming of something very exciting and new - and just a little bit different." 

(Image: Trinity Mirror)

A call has also gone out for local extras aged over 50.  Game of Thrones, described as the world's most popular TV show ever, has filmed extensively in Northern Ireland and also used sites in Iceland, Croatia, Spain, Malta, Morocco, Canada and the United States. 

On June 8 this year HBO, who also made hit shows The Sopranos and The Wire, commissioned a pilot to a Game of Thrones prequal. 

HBO has given a pilot order to a prequel series from writer Jane Goldman, one of five Game of Thrones projects that have been in the works, the Daily Post reports. 

Thomas Hopper, the architect of Penrhyn Castle, is understood to have also been the architect behind Gosford Castle in County Armagh, in Northern Ireland. 

Gosford Castle was used in Games of Thrones in season one in which it was used as the backdrop for when the character Rob Stark executed Richard Karstark for treason.  It was also used to film scenes in season three for the infamous Red Wedding episode, in which several main characters were killed. 

Part of the castle has recently been put on the market. 

(Image: David Powell)

Two other feature films have been shot partly in Wales over the summer.  In July the comedian Eddie Izzard shot part of his new movie in Llandudno called Six Minutes to Midnight, which also stars Dame Judi Dench and Jim Broadbent. 

In June, a film crew shot scenes for a major new movie version of the Secret Garden starring Colin Firth and Julie Walters at Bodnant Gardens. 

The film reunited Heyday Films and Studiocanal, after their collaboration on the Paddington movies. 

Wales Screen Business Wales refused to comment whether HBO were filming a Game of Thrones prequel.