It is a chilly March day and a group of “Celtic warriors”, blue facepaint gleaming, pull their shawls closely round their shoulders to ward off the cold. The task of the moment is softening daub, a mixture of horse dung, straw and clay which they will put on the skeleton frame of an unfinished round house. It’s a messy business. In the summer they squashed and softened the daub with their bare feet, but today they’re wearing their Wellingtons.

The would-be fierce warrior Celts are seven to eight-year-olds from Rhosymedre Junior School in Clwyd taking part in a new activity offered by the Groundwork Trust at its Legacy Environmental Education Centre near Wrexham. Developed in partnership with the National Grid Company, the centre has a brightly-painted classroom with an office, several nature trails, a pond with dipping platforms and the round house which is built close to a mass of metal pylons, buzzing wires and yellow warning signs - the Legacy sub-station.

It’s a bizarre juxtaposition. “Some people think it’s a pity that the sub-station is there,” says Julia Baron, executive director of Groundwork Wrexham Maelor. “But I think it provides a good contrast between the old and the modern.” The site has particular Celtic resonances, she says - it’s within a few hundred yards of Offa’s Dyke, an earthwork built by the King of Mercia to mark the frontier established by his wars with the Welsh.

The round house was built by a group of pupils from the nearby comprehensive, Ysgol y Grango, many of them persistent truants. As part of a special project they researched Celtic life - in particular the techniques used to build a Celtic hut. They became so involved that “it was noticeable that they tended to come to school very regularly on those days when the project was scheduled”, says Julia Baron.

Once the house was there, Groundwork wanted to use it. Hands-on projects on the Celts available in other parts of Wales - notably at Castell Henllys, the Pembrokeshire site of an Iron Age fort - were nowhere to be found in northern Wales.

After some research at Castell Henllys and Celtica, an ambitious high-tech project in Machynlleth, a week for schools organised by Groundwork around St David’s Day proved popular. It is now the most booked of the many theme days the centre offers.

The day begins with a video on Celtic life, a Welsh story and a look at a small collection of artefacts: copies of shields, chainmail, helmets, pots and ironwork. The children try on the helmet and feel the enormous weight of the chainmail and sword - warriors usually carried knives rather than swords for this reason.

Ex-teacher Anne Frost, who runs the centre’s events, bases the day round a tale from the Mabinogion, a collection of medieval folktales which draw on Celtic and Norman legends. Culhwch, chief of a tribe wants to marry Olwen, but her father won’t give permission unless he undertakes certain tasks, including bringing back the head of a giant and killing a wild boar. But Culhwch goes missing. Dressed in shawls and with their faces painted, the pupils go on “a magical journey into the past” to search for him.

First they have to learn the skills of Celtic children and begin with the construction of the round house by weaving young saplings through posts, a technique they also use to make a fence. Later they split into groups to take turns at different activities: one makes vegetable soup; another decorates large cardboard shields with Celtic shapes; another does weaving on small frames made from saplings and a fourth uses simple spindles to spin sheep’s wool. In the afternoon, groups make small finger pots and bake yeast-free bread.

Then comes the making of wattle and daub, after which the whole party practises spear-throwing: screaming loudly as, in true fearsome warrior fashion, they propel wooden missiles at a sheep skin slung from a tree.

“It’s the hands-on aspect which works so well, with the variety of experiences,” says Rhosymedre’s head Tom Nicholls. “You can talk about wattle and daub, but they have no real idea; by doing it they understand. By picking up the sword, they realise why warriors normally carried knives. It also gives them an insight into how difficult life was in those times; how everything had to be made.”

The centre is available to organised groups throughout the year, including school holidays. For further information contact: Centre Manager, Legacy Environmental Education Centre, Legacy, Clwyd LL14 4HY. Tel: 01978 843404.