Look to sea from Portsmouth harbour and your eye is inevitably drawn to the three strange stone blocks on the horizon. While the waves rise and fall, the yachts heel and powerboats race, these granite bastions sit solid and immovable, immune to the changing of tides or passing of centuries.

Even if you take a boat out to get a closer look, their blank, circular walls of stone and wrought iron give little hint of their purpose or what they contain. You might guess that they are oil rigs before deciding they are clearly too old, or perhaps Victorian lighthouses, were they not much too burly.

The one function you would never imagine for such forbidding structures is hospitality but last week Spitbank Fort, the closest to shore of the three, opened its doors as Britain’s most unusual luxury hotel. I was to be its first guest; after a 15-minute crossing, my boat pulled up at the landing stage and I climbed the metal steps to its imposing entrance.

Spitbank was built at vast expense between 1867 and 1878, alongside its two neighbours, No Man’s Land Fort and Horse Sand Fort, to guard the entrance to Portsmouth harbour, home of the Royal Navy since the reign of Henry VIII. In the event, the feared attack from the French did not materialise and the forts never fired their cannon in anger, becoming known as “Palmerston’s follies” after the prime minister who had ordered their construction.

Spitbank was finally sold off by the Ministry of Defence in 1982 and, after spells as an occasional restaurant and party venue, it was bought in October 2009, sight unseen, by Mike Clare, a genial British entrepreneur best known as the founder of bed retailer Dreams (a business he later sold for a reported £230m). Clare showed me round his purchase in July 2010 and, as we toured its waterlogged basement and crumbling concrete gun emplacements, he set out his ambitious plans for turning it into a unique hotel. The potential was clear but so was the scale of the job, and the opening date he proposed, Easter 2011, seemed optimistic.

It was. Launch dates came and went; my visit to see the new hotel was postponed three times (including one last-minute cancellation because of an unpleasant-sounding emergency with the sewage treatment plant). When I asked Mark Watts, the general manager, to identify the key factors in the delays, he said simply: “Being in the middle of the sea.”

One of the bedrooms

The cost of the works rose to £2m, on top of the £1m purchase price, or a total of £375,000 for each of the eight double rooms. So when Watts pushed open the heavy wooden door and ushered me inside, I was expecting a grand transformation.

I was greeted by a large puddle on the stone floor, just where it was on my last visit. As we tiptoed around it, Watts muttered something about high waves splashing through the door the previous night, promising it would be fixed immediately. He wasn’t making this up – the waves had prevented all boats reaching the fort the previous day, thus leading to the latest postponement of my visit.

Forecasts of more high seas to come, and the prospect of my getting trapped on the fort, meant my overnight stay was downgraded to lunch and a day visit.

But things improved. A door off the hallway opens to the Victory Room, a bar area with room for 60 people, full of low sofas and leather armchairs, with vaulted brick ceiling and stone flagged floors. It’s a relaxing space, with light pouring in from both sides but, most importantly, the modern furnishings are low-key, almost anonymous, so it is the fort’s original features that catch the eye. Colossal iron hooks protrude from the ceiling, once used for pulleys to position the guns and hoist shells. In one alcove are the eight basins where the soldiers washed, in another is an ancient fuse box. There are more hooks in the walls where they hung their hammocks and curved iron rails for manoeuvring the cannon are set into the floor.

Originally the gun emplacements ran right around the fort in an open gallery, but partitions have been introduced to divide it into bar, restaurant and bedrooms, which are surprisingly large and airy for a 19th-century fort. The gun ports have been converted to windows, which open so you can fall asleep to the sound of the waves. All the rooms are ensuite; some come with wooden ammunition hoists beside the dressing tables.

The rooftop has a plunge pool with views over the Solent to the Isle of Wight

At the centre of the fort is a circular courtyard, with steps down to the lower level and up to the rooftop. Downstairs the storerooms have been converted into gym, games and TV rooms while the central ammunition store has become a wine-tasting room – with 15ft-thick armour-plated granite walls, this is the place to cut a secret deal away from prying eyes.

Upstairs are three sunbathing decks, a slick modern bar/lounge with floor-to-ceiling windows, a barbecue and firepit area and the pièce de résistance, a rooftop gun emplacement converted into a circular plunge pool, with adjacent sauna and views over the Solent to the Isle of Wight.

In places, the decor doesn’t quite fit – the Admiral Churchill suite has silver furniture and a white leather Chesterfield with diamanté studs – but overall Clare’s team has done an impressive job, clearly motivated by a desire to preserve the building rather than maximise its profitability.

“If you had gone to a bank with this as a business plan, they would have laughed!” says Watts, arguing the project would have been impossible without a single, committed, private owner. The commercial drawbacks are easy to see: the fact there are just eight rooms, the building’s status as a scheduled ancient monument and the strict regulations that entails, the need for its own sewage treatment plant and diesel generators, and for staff and supplies to be delivered by boat. Then there’s the weather – Watts insists that on average boats will be unable to land on only five days per year, but accepts that even this will make it a hard sell in the lucrative wedding market. You can’t help wondering if it is more of a novelty than a serious business, to one day be known as “Clare’s folly”.

But Watts is already reporting strong bookings, particularly from the corporate sector, both for product launches and from companies who want to entertain staff and clients in lavish surroundings, but not be seen to be doing so. “People have become very coy about where they are spending their money,” he says. A private departure lounge has been created on the mainland at Royal Clarence Marina, on the quieter Gosport side of the harbour, to ensure guests can embark discreetly.

Clare must be confident. Rather than wait to see how Spitbank fares, last month he snapped up Horse Sand and No Man’s Land (for an undisclosed sum), both of which are three times larger that Spitbank. Horse Sand will be preserved as a museum and opened to the public, while No Man’s Land will be renovated and launched in 2014 as a 30-bedroom hotel complete with an indoor swimming pool and a spa for both residents and day visitors. It looks like the start of a significant new tourism industry amid the waves.

After lunch I went back to the roof to explore some more. On one hand, it isn’t hard to picture Spitbank as a rich man’s prison, where the privileged will idle away the hours with nothing to do but eat and drink and stare wistfully back to land through the numerous telescopes. Even swimming in the sea is forbidden due to dangerous currents.

But the reality is that most visitors will be here for a night or two, not a week, and will be busy fishing for mackerel and sea bass, taking trips to the Isle of Wight’s beaches in the high-speed RIB that is moored alongside, popping over to the spa on No Man’s Land, or simply sitting and taking in the view. I watched frigates on exercise, a rescue helicopter hovering overhead and a passenger hovercraft rising up the beach on the mainland. Clouds whipped overhead and the water seemed to change colour with every minute, passing from turquoise to slate grey and everything in between. It was hard to tear myself away.

It may not have palm trees or white sand but this private island has one other key selling point. A direct train link means that a little over two hours after turning away from the view, I was back in central London, the faint smell of sea salt still lingering on my clothes.

Spitbank Fort (www.clarenco.com) sleeps up to 18 and costs from £5,500 per night. On certain dates double rooms can be booked separately, from £800 full-board

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