Along England’s wild northern frontier, Hadrian’s Wall crosses a sublime, historic landscape that is still giving up the secrets of the region’s turbulent past.

There’s something about long-distance walking that puts you in a slightly altered mental state, but I’m still surprised when I see a Roman legionary coming towards me above Walltown Crags.

The man’s breastplate catches the light as he makes his way up the steep slope. Behind him, Hadrian’s Wall stretches away into the mist. On the northern side of the frontier – barbarian country – there are a few animals grazing, but the farmland looks wilder and more untamed than the pasture to the south.

Even for someone fit, the climb uphill is testing. At the summit, the legionary gasps that he’s marched from Segedunum (present-day Wallsend in Newcastle), before pausing to catch his breath. I fumble for my schoolboy Latin and can’t remember any: the word I’m looking for is ‘cur’ – why?

‘I like difficult things,’ he says. ‘I’m not being unkind, but anyone can walk Hadrian’s Wall in that.’ He gestures at my anorak and sensible shoes.

Richard Parker is the legionary’s name and he poses gamely for pictures with his plastic sword. There are lots of good reasons to walk Hadrian’s Wall: it crosses the entire width of England; the landscape is spectacular; it’s an archaeological site of supreme importance; it’s a physical challenge – though the truth is there’s no reason to do it bare-legged and wearing a heavy helmet on your head, even if, like Richard, you’re doing it to raise money for a neonatal unit in Plymouth.

He says goodbye and sets off towards Barrow-in-Furness, past the wall’s western end. He plans to be there in four days.

As I watch him go, with his red nylon cape flapping out behind him, he looks more John Cleese than Julius Caesar, and it strikes me that many of our associations with ancient Rome are comic. But walk the path for any distance, and as the wall unfolds in front of you, it reveals a past that was also epic, poignant and strange.

I’d begun my walk the day before, heading east from Lanercost in Cumbria. It’s my plan to walk the 40-mile stretch to Heddon-on-the-Wall in Northumberland over four days. I set off early in the morning as a man called Maurice brings out the honesty box from his shed to set beside his bag of home-grown apples. Munching on one of Maurice’s apples as I walk past the first standing remnant of the wall at Hare Hill, I find it hard to reconcile the peaceful atmosphere around me with what I know of the region’s history. Ripe sloes and crab apples line the path that runs along green pasture where sheep and cattle graze happily in the soft rain.

Hadrian ordered the construction of his wall in 122 AD; ‘to separate the Romans from the barbarians’, his biographer wrote. It stretched in its heyday from coast to coast, with a pair of turrets and a ‘castle’ or small fort every mile, and deep earthworks on either side. Larger forts, some of which predated the wall, garrisoned the troops and housed their dependants.

Until antiquarians began to realise the significance of Hadrian’s Wall and fought to preserve it in the 19th century, it was pillaged by locals for building material. Along its length, farmhouses that have been built out of Roman masonry still sit next to denuded sections of the wall. All the same, according to Richard Hingley, a Durham archaeologist who’s writing a biography of the wall, locals never entirely forgot its importance. It helped define the border between England and Scotland. And as early as the 16th century, local landowners were collecting stones from the wall to display in their gardens.

Nowadays, much of the wall is gone; sections of it are no more than dots and dashes of stonework. At the eastern edge of the route, slabs of wall turn up incongruously in Tyneside housing estates. Every now and again, you come upon a stretch that is sufficiently intact to give you a sense of what it must have been like during the centuries when it was Rome’s northern frontier. Ruts cut by hundreds of years of wagon wheels can be seen in the stone gateways of forts. Here and there are stones carved by the soldiers to commemorate the completion of their work: ‘the cohort of Primus Pilus built this’ scratched into a stone, or a phallus etched into a limestone block to propitiate a forgotten god.

Six miles east of Lanercost, I reach the tranquil bend of the River Irthing, where a new bridge carries the long-distance footpath to the hamlet of Willowford. The bridge, lowered into place in 2001, is the first to span the Irthing since Roman occupation.

The remains of the abutment that supported the Roman bridge stand a hundred yards from the present course of the river. It’s odd to think that, 1,600 years after the Romans left, the infrastructure they built has only just been equalled.

I begin my second day plodding up the path from Walltown quarry to embark on the most dramatic stretch of the wall. This section was shaped by forces even more ancient than the Romans. Between Walltown and Sewingshields Crags, the wall sits astride a natural geological feature – the Whin Sill, a big prow of dolerite that towers over the moorland in front of it.

This is the toughest, most exhilarating walking of all. You hug the ridge of a huge inland cliff, looking out over a sea of moorland and trees towards the Scottish uplands. It feels as though the wall itself has been energised by the extraordinary landscape; it seems to defy both gravity and common sense as it runs dizzyingly close to the edge of the cliff, switchbacking over the peaks and dips of the escarpment. Way off in the distance, the sheep look like clumps of tiny white mushrooms.

It’s impossible to walk Hadrian’s Wall and not become fascinated by the Romans and Britons who lived, worked and died here. They come to life with surprising vividness. In the 1970s, archaeologists excavated the remains of a Roman rubbish tip in the fort of Vindolanda. The airless condition of the site’s dense soil means that organic matter has been preserved: leather, wood and textiles have survived almost two millennia in the Northumbrian earth. There is something strangely moving about the familiar objects that have turned up here: beautifully worked shoes, a chair leg, a hairnet, a child’s wooden toy sword.

The excavators also discovered wooden fragments bearing Latin handwriting. What have come to be known as the Vindolanda Tablets include a letter of complaint, roll-calls of troops, a derogatory reference to the locals as ‘brittunculi’ (‘Little Brits’) and, perhaps most poignantly, a 1,900-year-old invitation to a birthday party. The invitation is written in part by a professional scribe but includes a brief message by the hostess of the party. ‘I shall expect you, sister. Farewell, sister, my dearest soul, as I hope to prosper, and hail.’ They are the among the earliest examples of a woman’s handwriting in Latin.

Vindolanda today occupies a surprisingly big site about a mile to the south of the wall. The grey stones of Roman foundations – homes, temples, bath-houses – stand out against the green Northumbrian grass, evidence of a complex and thriving community.

Last autumn, the dig at Vindolanda brought a more unsettling discovery. Under the floor of a barracks, where it had lain since about 250 AD, archaeologists found the skeleton of a child. They called in Dr Trudi Buck, an anthropological expert from Durham University, to try to determine the child’s identity and manner of death.

Dr Buck tells me that it’s highly unusual to find human remains inside one of the Roman forts. ‘It was actually illegal to bury people there, so it was certainly a clandestine burial,’ she says.

Her preliminary measurements of the bones suggest that the child was aged between eight and 10 years old, but its gender is still uncertain. Further analysis of the DNA and chemical composition of the bones is due to take place and may help to shed more light on the child and its fate.

While it’s tempting to idealise Roman civilisation with its baths, straight roads and military discipline, clearly things are more complicated than that. Few wish to get carried away with speculation, but there is something lonely and strange about the child’s secret internment.

The site’s museum director, Patricia Birley, suspects something unpleasant took place. She says the body’s hands were tied, and there were no clothes buried with it. ‘Societies change, civilisations change, but human nature hasn’t changed,’ she says.

After the Whin Sill, the return to ground level comes as an anticlimax. It’s steady going from here until Heddon. I share the almost empty path with cattle and sheep. A big chunk of wall runs through the middle of their pasture. I skirt behind it to avoid what looks like a bull; two millennia on, the wall is still serving a useful defensive function.

The Roman soldiers left Britain in 410 AD. The confidence of their predecessors seems to have evaporated. ‘In 122 AD, the Roman empire had the northern frontier largely under control,’ Hadrian’s Wall historian Richard Hingley had told me. ‘By the early fifth century, things were seriously running down, and the priorities of Rome were far away from Britain.’ New threats at home made Rome scale back its imperial commitments. They made an orderly retreat. At Brocolitia Fort, the departing soldiers blocked up gateways to render it unusable. Over time, grass has carpeted the remnants of its walls with green.

In the car park by the fort, Ant Wright serves espresso from his stall and enlightens his customers about the more arcane aspects of Roman religion. Just behind him lie the remains of a temple to Mithras, or Mithraeum. It is one of the most complete in Britain. ‘The Romans believed Mithras was born on 25 December, out of a rock or an egg,’ says Ant. ‘The more you look at it, the more parallels there are with Christianity.

There is certainly something eerie about the excavated temple. Although roofless, it feels oppressive: a tight rectangle of low walls and odd sculptures. Mithraeums were small, designed to evoke the atmosphere of the mythical cave where Mithras slew a bull and new life sprang from its blood. By the door is the place where adepts underwent ordeal by fire as part of their initiation; three altars – copies of the originals – stand at the end.

Back in the car park, I ask Ant when the Mithraeum was discovered. ‘Ask her,’ he says. ‘Her dad found it.’

Jennie DuCane is walking past with her dog, Wesley. ‘Actually, my father’s dog found it,’ she says, and tells me the story.

In 1949, Jennie’s father Richard bought the land on which the Brocolitia Fort sits. The turf had been cut in preparation for raising fence posts. Richard’s dog, Adam, pawed at the ground and revealed a piece of stone that had clearly been shaped by hand. ‘It didn’t take my father long to realise it was an altar,’ says Jennie.

Even in its reduced state, a shred of mystery still clings to the temple. I add some change to the offerings on the central altar and set off on my way.

To walk in the footsteps of legionaries and the Roman elite, with their strange gods and oddly familiar habits, is both exhilarating and humbling. It is a peculiar thrill to walk the same hills they did and look at the same views. On the land around the wall, the wild chives they brought with them to season their food still grow.

If there were no Roman remains, this place would still be extraordinary. There’s a spaciousness and sweep about the countryside here that feels un-English.

On my last day, I revisit my favourite sections of wall, using the bus to hopscotch between them. The mist has rolled back and there is golden light on the crags and views in every direction: the Pennines, the Lakeland Fells, the Cheviots to the north. In the woods above Housesteads Fort, the butterflies come out and pheasants flop off into the bracken. Beneath the Whin Sill, there is a flash of red and I think it’s another legionary’s cloak. I slither down the hill to see who’s walking in fancy dress this time, but it’s a huntsman in a scarlet coat, giving his hounds a run. I chase after him, but he is soon gone. Instead, I’m rewarded with the dramatic view south – the wall from barbarian country, still rolling imperiously over the towering crags.

The article 'Walking Hadrian’s Wall' was published in partnership with Lonely Planet Magazine.