From the top of the 440m-tall hill of Penycloddiau, the Clwyddian Range of northern Wales spreads out before you.

It is easy to think that the beautiful view is the real attraction here. After all, unless you are an archaeologist, the roll that borders Penycloddiau's summit – like a heather- and gorse-topped bumper – might seem like a natural phenomenon.

It is not.

Advertisement

Think you know the British countryside? Think again!

There’s much more to the British countryside than you’d believe, take 45 seconds out of your day and uncover the hidden secrets of beautiful Snowdonia.

 

Press play on Quick Britain

Fiona Gale, archaeologist for Denbighshire County who spearheaded the Heather and Hillforts project, stops on the dirt path. She points to a bump in the land next to us. "Presumably, there would have been a gate here, and a palisade-like fence all the way around," she says. The other roll of land, just a few feet away? "A guard chamber."

These features are far from incidental. They are man-made, built around 3,000 to 2,500 years ago.

But nobody knows what these structures, dubbed "hillforts", were used for. Were they for defence, as their name suggests? Settlement? Storing grain? Showing off?

Take that "guard chamber". It could indeed have been a place where a military guard stood to protect the enclosure. But it could also have been a shrine: a place for those entering or leaving to pray or give thanks.

In an area that measures some 150 square miles (389 sq km), there are about 30 hillforts

"The language of these things was established in the early 20th Century when we were fighting a lot of wars: hillforts, guard chambers," Gale says, a little ruefully. "We're stuck with these terms. But I think they were much more complicated than just being military or defensive."

Enclosing an area of 21 hectares (51.8 acres), Penycloddiau is the biggest hillfort in northern Wales

The wall that surrounds it, now buried beneath dirt, heather and gorse, was some 13ft (4m) thick. Stone-faced on the inside and the outside, with a rock-cut ditch below and likely with timbers on top, it would have towered an imposing 33-39ft (10-12m) tall.

The structure would have been visible for miles. And it would have taken a great deal of work and organisation to build. The timber alone needed would have required around 170 hectares (420 acres) of woodland.

Yet as extraordinary as it is, Penycloddiau is hardly the only hillfort in this area – or in Britain.

Similar hilltop enclosures are particularly common in southern England, less so in Wales or Scotland. But this section of Wales is thick with them. In an area that measures some 150 square miles (389 sq km), there are about 30 hillforts.

It would be very difficult for a group to defend a hillfort

This adds to their mystery. It is a surprising number for a region that, at the time, did not have a particularly dense population.

So why were they built at all?

The name hillfort, of course, suggests the enclosures were built for a military purpose. But there are problems with the simplicity of this explanation.

"It would be very difficult for a group to defend a hillfort, especially one the size of Penycloddiau," says University of Oxford archaeologist Gary Lock, who is leading an excavation at the nearby Moel-y-Gaer hillfort. "How many hundreds and hundreds of people would you need to defend that? Then, at the same time, you need another great group of people attacking it. There just isn't that level of population."

Warfare in small-scale societies tends to be heavily ritualised, full of rules and conventions

There are other flaws with the idea, too. For example, few of the area's forts have their own water source.

Meanwhile, hardly any weapons have been found. This is true for weapons of that time in general – it is much more common to find weapons from the previous era, the Bronze Age.

One reason might be that, by this period, the preferred weapon seemed to have been slingshot stones. These have been found in hillforts, but generally not in sufficient numbers to suggest a full-scale battle.

Still, the idea that an organised group would bring a cartload of stones up to an enemy hillfort seems a little absurd.

Ian Armit of the University of Bradford has tried to figure out how defensive these hillforts really were, by looking at more recent structures in other cultures.

At the end of the Bronze Age, this elaborate system of wealth and trade collapsed

He has found that warfare in small-scale societies tends to be heavily ritualised, full of rules and conventions. Siege warfare "is not something that often happens", he says. "It's seen as cowardly or unfair."

In that context, even a lack of water can be explained. In fact, Armit points out, when Julius Caesar was attacking Iron Age hillforts in Gaul, the Roman general wrote about how his soldiers would attack the enemy when they left their forts to get water.

But if warfare at the time meant skirmishes, not sieges, why build a hillfort at all?

To understand that, it is important to understand the shift from the Bronze Age to the Iron Age. 

From about 4,000 to 2,800 years ago in Britain, bronze was the primary way to build alliances and trade networks across communities. Society was more organised, warfare tended to happen on a grander scale, and the weapons were more glorious. That may be why it is easier to pick these weapons out of the archaeological record.

But at the end of the Bronze Age, this elaborate system of wealth and trade collapsed.

It's probably to do with people coming together to ensure that crops don't fail

The reasons remain mysterious. We know that the climate had been deteriorating in parts of Britain. We also know that iron – probably introduced to Britain by continental Europeans – was replacing bronze as the metal of choice.

"It's a bit like a bank crash," says Niall Sharples of Cardiff University. "They get to the state where everything revolves around bronze and the exchange of bronze. Then external factors cause that to collapse, no one has faith in the system and they have to find another way."

The crisis is thought to have ushered in the change to what we call the Iron Age. In Britain, this lasted from roughly 2,800 years ago to the Roman invasion 1,900 years ago.

At about the same time, hillforts begin to be constructed in earnest.

A few reasons may help illustrate why. First, in the absence of those long trade routes and of exchanging bronze as a way to secure friendship, society itself became more fragmented. No longer able to depend on far-flung allies, groups became more self-sustaining and insular.

Meanwhile, the effects of the changing climate made farming a riskier business. As a result, people may have pooled resources; for example, storing their grain together. In fact, storage pits have been found in a number of hillforts.

I think they were much more complicated than just being military or defensive

"I think the origins of hillforts probably do have a lot to do with what happens at the end of the Bronze Age [in terms of climate] – which we know had quite an effect out here," says the University of Liverpool's Rachel Pope, who is leading the excavations at Penycloddiau. "It's probably to do with people coming together to ensure that crops don't fail, to ensure that animal herds survive."

And third, no longer able to flash their power with bronze, a tribe or group might have turned to a different kind of status symbol: a hilltop structure – one that required significant manpower, resources and, of course, that could be seen for miles.

By making a statement of prestige and group identity, as well as being a place to store grain, a hillfort may have helped people accomplish several goals at once. If it could keep them safe from potential enemies, so much the better.

Hillforts also may have doubled as settlements – though not necessarily year-round.

Once again, archaeologists run into a vexing lack of artefacts showing this. But that does not mean the settlement theory is wrong. In the Iron Age in the area, people did not use pottery. And in the acidic soil of the Welsh hills, metal and bone both disintegrate. This all makes hard evidence hard to come by.

We can see that it's certainly more to do with agriculture than defence

As a result, the most you might expect to find is a burn pit, which was turned up at Moel y Gaer, Llanbedr in 2009. Or something handmade but stone, such as a spindle whorl showing weaving activity – which was exactly what the team led by Lock and his colleague John Pouncett found at Moel y Gaer, Bodfari. (Moel y Gaer means "fortress hill" in Welsh).

Then there are the imprints left from buildings.

On Penycclodiau, Pope and her team identified 82 potential roundhouse platforms, some of them around a spring. They are currently excavating one of them. "The traces of occupation are often quite fragile," Pope says. "We haven't found anything as straightforward as a hearth yet. But we do have postholes in the interior."

At this point, Pope thinks that settlement at Penycclodiau was mainly seasonal.

"When we look at the origins of hillforts in Britain, we can see that it's certainly more to do with agriculture than defence, that it's as much about bringing animals into space as people," she says. "Settlement in many cases – not in all cases, but in many cases – seems to be relatively temporary. The houses aren't terribly steadfast."

That is part of the fascination I have with hillforts: the not knowing

Will we ever know for sure why hillforts sprang up across Britain? Probably not.

But the possibility remains that the ongoing excavations in the Clwyddian Range will yield more clues. Many of the hillforts here have never been excavated, or were only studied decades ago, when archaeologists were much less systematic than they are today.

"The more we do and the more carefully we do it, the more we'll get answers, but we'll never get all of them," Gale says. "That is part of the fascination I have with hillforts: the not knowing."

This story is a part of BBC Britain – a series focused on exploring this extraordinary island, one story at a time. Readers outside of the UK can see every BBC Britain story by heading to the Britain homepage; you also can see our latest stories by following us on Facebook and Twitter.