Extending to approximately 2.49 acres, Thorne Island has recently gone up for sale, with the owner seeking offers in excess of £3 million ($4 million), according to the listing on the Strutt & Parker website.

The covered rooftop bar is one of the property's highlights. (Photo Credit: Strutt & Parker via CNN Newsource)

The fort’s highlights include a helipad, a covered rooftop bar with a games room and a sea-view office.

Originally designed to house 100 men, the restored property can today sleep up to 20 people in its five plush bedrooms, while other living spaces include grand dining rooms and terraces sheltered by barrack-style walls.

One of the property's five bedrooms. (Photo Credit: Strutt & Parker via CNN Newsource)

The fort was built on the craggy outpost between 1852 and 1854 as part of a wider plan to strengthen national defenses against a possible Napoleonic invasion, according to the property listing.

But over time the Welsh island’s military role faded and the fort has served as a hotel and family home since the island was first sold in 1932.

Its current owner, British tech entrepreneur Mike Conner, bought the island in 2017 for £500,000 ($670,000) after seeing a video about it on YouTube, he told CNN.

Back then, the fort – protected by law due to what’s known as a Grade II listing – had no windows or utilities and was waterlogged, but Conner said he was fascinated and seized the opportunity to undertake a once-in-a-lifetime restoration.

The fort, which was built to defend against a French invasion, has undergone a major overhaul. (Photo Credit: Strutt & Parker via CNN Newsource)

Conner said he had “underestimated a lot of things,” including the effort involved in installing flushing toilets.

“There was a lot of stuff that needed to be done to make it happen,” he said, including digging trenches through rocks and installing a running water system.

It took his team 350 helicopter trips over two days to transport materials – from topsoil and building materials to heat pumps for central heating and hot water – to the island. Meanwhile, builders lived on-site in the original barracks for four years while reconstruction was underway.

Thorne Island, complete with a 19th century fort, could be yours for $4 million. (Photo Credit: Strutt & Parker via CNN Newsource)

Part of the reason it took so long was that the fort was in bad shape when Conner took it over.

Sandblasting the walls to reveal the original stone alone took six months, while the granite walls in parts were so saturated with historic moisture that it led to flooding, according to a press release sent to CNN.

Conner – the founder of Appsbroker, which later became Qodea – said the overhaul, which has only been completed this year, cost more than £2 million ($2.7 million).

The substantial investment included a £300,000 ($400,000) outlay on renewable energy – such as solar panels and battery storage – and a biodigester system for wastewater, which allow the off-grid fort to be completely self-sufficient, he said.

Conner, 52, said he has a degree in industrial design, adding that he had embarked on the project as a bit of a challenge.

Tech entrepreneur Mike Conner said the undertaking had been part of a "mid-life crisis." (Photo Credit: Strutt & Parker via CNN Newsource)

“It was quite a healthy mid-life crisis by some of my friends’ standards,” he said.

The fort, which is only accessible by sea or air, had to be renovated to withstand salt air and storm-force winds.

Conner, who has spent time in the renovated fort with his family, believes it could be a perfect venue for luxury retreats and celebrations or for someone who really wants to get away from it all.

As for him, he’s on the lookout for a new challenge. “I’ve enjoyed the journey and I think I would like to find another basket case,” he said.

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