Wandering among the chestnuts, oaks and Scots pines on Chatley Heath in Surrey, you arrive without warning at a place where the branches part and you find yourself looking up at a slender octagonal tower. Its regular red bricks are a surprise after the tangle of winter-brown trees and there is something of the fairytale about it. It would be tall enough to trap Rapunzel and, like Sleeping Beauty’s castle, it seems to have been enclosed by woods that have grown up in the years since it was forgotten. Stranger still, if you live in southern England, you have almost certainly passed within 150 yards of its front door, without ever even knowing it was there (more on that little trick later).

In fact, such whimsy is way off the mark: this was once a cutting-edge piece of military technology, a key node on the pioneering information superhighway of its day. In 1815, just 11 days after the battle of Waterloo — in Wellington’s assessment, “the nearest-run thing you ever saw in your life” — Parliament passed an act enabling the Navy to acquire land for a new early warning system. It would connect Admiralty House, the Navy’s command centre in Whitehall, with the docks at Portsmouth, via a chain of 14 relay stations. Messages were to be passed along the line via a system of semaphore recently devised by Rear Admiral Sir Home Riggs Popham, originally for use on ships.

The Chatley Heath tower was completed in 1822 and by 1824, the entire line was up and running. Each station had a roof-mounted mast with two moving arms, the positions of which indicated letters, numbers and a few frequently used words. In clear weather, a message could be transmitted from London to Portsmouth in as little as 15 minutes, rather than the eight hours it might take on horseback.

Many of the stations were simply one- or two-storey houses with a mast on the flat roof, but at Chatley Heath the surrounding topography required a five-storey tower with a 40ft-mast on top. The system worked so effectively that the Admiralty commissioned a second line, branching off at Chatley Heath and continuing west to the naval base at Plymouth.

‘View of the Admiralty’, painted in 1818 by Thomas Shepherd; the semaphore mast is visible on the roof

Today this, the only surviving semaphore tower, has a less strategic role. Following a major renovation by the Landmark Trust, it will be available for rent as a holiday home, starting this coming weekend. While the majority of the Trust’s 198-strong portfolio of historic buildings are in rural and sometimes remote locations — including a fisherman’s house on a secluded bay in the Highlands, a lighthouse on Lundy Island, and a Welsh cottage only accessible by steam train — the Semaphore Tower is altogether closer to hand, only 19 miles from Trafalgar Square as the crow flies.

And what you don’t see in the picture above is the M25, London’s orbital motorway, which was built in the 1980s and now runs only 140 yards from the tower’s front door. The trees, and the fact the carriageway is in a cutting at this point, mean the tower is invisible to drivers. Two-thirds of a mile to the north-west is Junction 10, where the M25 meets the A3, an interchange passed by 294,000 vehicles per day according to Highways England. Thankfully the bedrooms are so effectively double glazed you can’t hear any of them.

The approach is surprisingly bucolic. Guests will arrive via a lane through green fields, at the end of which they find a gate. They unlock it, then drive up a track and to the property — fairly standard Landmark Trust procedure, except here the track emerges from hedgerows to cross a narrow bridge over eight lanes of thundering traffic, before burrowing back into woodland.

A room inside the refurbished Semaphore Tower. The building was completed in 1822

And yet, inside its picket fence, the tower remains something of a time capsule, oblivious to the new roads, and the commuter towns and villages that have expanded in a circle around it. Lieutenant Edward Harries, the first superintendent of the tower, would find the building’s exterior little changed from when he arrived with his wife and five children in 1822. Inside is a different matter. Harries wrote repeatedly to the Admiralty to complain about the leaks and the spartan living conditions. He would have been dazzled by the comforts afforded by the renovation, not least the running water, electricity and spacious modern bathrooms.

But there is still a period atmosphere — no television, radio or microwave — and the rooms have the vaguely maritime feel of a naval building or customs house. There are ship’s chests, big brass lamps and bedsteads, and a subtle tarry smell from the freshly lacquered floorboards. Framed prints show ships and sea battles; the bookshelf has a collection of Nelson’s letters to his wife, Hornblower and Jack Aubrey novels and numerous guides to signalling and semaphore. In the kitchen, bespoke units and a worktop curve around three walls of the tower, the craftsmanship worthy of any master shipwright.

To the south and west, away from the roads, the woods and heathland are much as they always were, still an important habitat for damselflies and dragonflies, the nightjar, hobby and Dartford warbler. The hum of the traffic quickly dissipates as you stroll the sandy paths, and beyond are the hills of the North Downs.

A path through the woods in the surrounding land. T

All the stations were staffed by navy lieutenants close to retirement (Harries was 58 when he took up his post) but this was no cosy sinecure. For up to seven hours a day, the superintendent and his assistant had to stand with telescopes at the windows, checking the next mast up and down the line. The telescopes were not to be left unmanned for more than two minutes and if fog made it impossible to see, the operators would post a signal showing that communication was interrupted. Crews were suspected of sometimes posting it just to get a break.

London “pea soupers” often blocked messages getting from the Admiralty even through St James’s Park to the first station at Chelsea, but for more than 20 years information was passed up and down the line. However, the big attack for which it was designed never came, and the technology was quickly superseded. The railways were coming, and with them the electric telegraph which could transmit messages instantly.

In 1847 the system was decommissioned, but the tower continued to be occupied until 1963, by gamekeepers and wardens of the surrounding heath. The building was perhaps rather too much of a time capsule for those later residents — even as the 1960s were starting to swing, the family living there were using well water and paraffin lamps instead of electricity. Ruby Ivey, who got in contact with the Landmark Trust during the renovation, lived in the tower as a young child in the 1930s and recalls collecting fir cones for fuel for the boiler, transporting them back to the tower in an old pram. When her mother drew well water to wash clothes “there were often frogs in it”.

“Dad was very good at telling stories,” she recalled. “In the winter evenings we would all sit around the old range in the light of the paraffin lamp and candles eating roast chestnuts as we listened. The wind through the trees used to make a wonderful sound, sometimes a gentle humming and sometimes a roar.”

When I visited, on a tempestuous morning earlier this month, the wind was still roaring in the branches. Sitting on the roof beside the great mast, you can’t help but think about how communications, and lifestyles, have been so transformed over two centuries. In the winter of 1823 Lieutenant Harries had gone without bread or meat for days because “the severity of the weather and the badness of the roads across the Heath” made it impossible to reach Cobham or Ripley (his requests to the Admiralty for a donkey to help get supplies were declined). Now there’s a charging point for electric cars, mobile reception is good, traffic speeds past just beyond the trees and London’s latest profusion of glass towers glints on the horizon.