This is Stradey Castle.
The Victorian Gothic mansion has remained virtually unchanged since it was built in the 1850s and extended 20 or so years later.
It has a striking exterior, beautiful stately rooms and stunning views across the Burry Inlet towards Gower. It has been home to the Mansel Lewis family since it was built.
And while the Grade 2*-listed house, set in a secluded location outside Llanelli, is open for wedding receptions, parties, concerts and photo shoots, very few people get to see the tunnels beneath it.
But Stradey Castle has a subterranean passageway that winds around the foundations of the impressive house, WalesOnline reports.
Known by the family as Guy Fawkes passageway, it is a creepy, brick-lined tunnel punctuated by twists and turns and underground windows, with spiders and moths lining the limestone walls illuminated here and there by the odd grill above. It's thought to have been there from the beginning.
Stradey Castle is one of the finest mid-Victorian houses in Wales and the present owner, Patrick Mansel Lewis, conducts heritage tours around the premises.
Patrick and his wife, Claire, started opening up the house for tours a few years ago to help finance a restoration of the building. People are only allowed access to the tunnels once or twice a year and only by appointment.
Claire Mansel Lewis, said: "It's known in the family as Guy Fawkes's passage. There is a story behind the name but it escapes me now; maybe it's because Guy Fawkes used an underground passage when he plotted to blow up the Houses of Parliament.
"It must have been installed when the house was originally built in 1850 because they made an artificial terrace to elevate the house. The ground floor was elevated for a better view. That would have been the obvious time to build it while the house was being built from the foundations up. It also doubles up as a damp proof system.
"It's very dark in patches with bits of ivy coming down, so it is a spooky experience walking through there, there's no doubt. Our three boys have always loved it and my parents-in-law had five grandsons and it was always a special place for them.
"There was a television programme made about the house called the Curious House Guest and during dinner we told the guest about the Guy Fawkes passage and everyone after dinner went down in their dinner jackets, with the camera crew, and walked around the passage. That was quite fun.
"We have undertaken a very expensive restoration project and we need to find ways of funding it. Our heritage tours last about an hour and we show people the outside of the house from the terrace and then bring them in, around the house, and then we offer homemade refreshments at the end of the tour. We try to make the family tours more fast moving and varied, with special interests for children.
"The Guy Fawkes passage is only opened up on special occasions, maybe once or twice a year. It's kept under lock and key as we are very conscious of security and the house and land is private; people can only visit by appointment."
Another way of raising money comes from living in a ready made film set.
She said: "Maybe twice a year we have a film crew of a kind come and visit. The Dr Who Christmas special in 2011 was filmed here, they filmed Heidi here, the Friday Download crew came and made a film here and more recently Dan Stevens, of Beauty and the Beast, and Julian Fellows, of Downton Abbey, came here for a day. David Suchet and Jack Whitehall came to film a couple of scenes for Decline and Fall by Evelyn Waugh."
But WalesOnline are among the first to film in the passageway itself, said Mrs Mansel Lewis: "Dr Who used quite a bit of the house but they didn't use the underground passage but the location scouts in Wales know about the passage, so who knows in the future."