Fight club gets hands dirty

"We kill each other off at fairs. Once you've slain one bloke, you move on to the next, but it's easy to get the wrong one. However much you train, the timing is crucial; there are a lot of wounds." Showing a healthy appreciation for the dangers posed by medieval mock combat, Mike Harrower hung up his latex sword several years ago, leaving the Clanranald Trust's fighting arm to hack gamely at each other without any help from him.

A project manager for Standard Life by day, Harrower now spends most of his spare time designing a motte and bailey medieval timber fort in the middle of the Carron Valley Forest for Clanranald, an educational trust. Most of the £250,000 cost of the project, which goes on site in April and will finish this year, comes from the trust hiring out its volunteers as extras and sword-stunters for epics like Braveheart. You may have seen Clanranald's driving force, a Goliath of a man called Charlie Allen, as the stuff-of-nightmares tribal leader in Gladiator who strolls out from the barbarians to hurl an unfortunate's head back into Roman lines.

As with many building projects, variations to the design of Duncarron Fort, 15 miles outside Glasgow, never seem to end. More unusually, new information can overturn even fundamental design decisions. Getting incontrovertible information on 12th century Scottish building practices is all but impossible. No descriptions or pictures survive, while the residues found by archaeologists indicate the materials used but not how they were put together. As findings from other sites emerge, Harrower doesn't so much tweak Duncarron as build it anew.

Bon motte
Take the longhouses. Harrower was all set to build them, Toblerone-fashion, from A-frames lodged in the earth 1m apart, as shown in the now-superseded model above. "I'd almost got a compliant A-frame when someone came in with a new site report," he groans.

The presence of vertical rather than angled post-holes at the site helped persuade Harrower to completely redesign the A-frames. Instead of sweeping all the way down to the ground, the A-frames now just form the roof, with eaves clear of the ground. A complicated system of diagonal bracing plus a vertical post at either end of each A-frame's crossbar holds the whole thing up, while new wattle and daub walls meet the roof at the eaves.

Surrounded by a ditch and palisade, the longhouses lie in an area (the 'bailey') that stretches 120m at its widest point; another palisade divides off a 10m-high mound (the 'motte') in one corner. While Harrower aims to use 12th century building techniques and materials as much as possible, the scale of the earthworks means bringing in JCBs for the grunt work. "We've got a 450m-circumference doughnut ditch that's 2m deep and 6m across," he says. "That's six figures of muck to be moved." Shifting all that with 12th century shovels would be some job.

Likewise, concrete pipes will divert a stream through the fort to power a forge. And rather than build dry-stone walls to stop the earth in the motte and palisade from slipping down, as authenticity demands, Harrower aims to stabilise the earthworks with hundreds of gabions – concrete containers filled with rocks.

What will rely on 12th century skills, tools and materials are the fort's timber structures – longhouses, roundhouses, wigwams, the forge, the palisade, and, on top of the motte, a chief's hut with a tower. Professional thatchers will roof the longhouses and roundhouses with heather interlayered with turf. Timber pegs will tie the top edge of the turf to the purlins – 2in-thick poles lashed to the edges of the A-frames with hemp ropes.

As well as holding the heather roof together, turf strengthens the wattle and daub walls around each longhouse. Laying the rolls of turf root face to root face helps to stitch wall and roof together as the roots continue to grow for a short time after the turf is cut.

We might just rip it all down and start again

Mike Harrower

Sourcing the stuff
With only a few months to go before the project goes on site, the sourcing of materials is now Harrower's biggest worry. Heather is no problem: there are plentiful stockpiles in the west of Scotland of harvested heather, which keeps for years and will give the roof a 70-year design life. And the hemp for lashing the purlins to the A-frames is a widely available crop in England.

Turf may be hard to come by locally, with local farmers unlikely to sell off their topsoil. Wherever it comes from, though, it ideally needs to be laid on the same day it is cut if the roots of the turf layers are to grow into each other.

And those A-frames are proving a problem again. Originally, Harrower intended to build them out of the larch trees that surround the site. He's since discovered that Japanese larch has a considerably shorter lifespan than European but has yet to find out which is the species planted in Carron Valley. If it's the wrong sort, he's thinking of importing oak from Europe. "It's getting scary, juggling at this late stage," he says.

Fortunately, the rest of the timber he needs is already on site – literally. Harrower is coppicing a stand of hazel around the car park to provide the poles that will be woven together for wattle. And clearing the partially forested site will produce enough spruce to build the palisade, the design of which, like much else in the fort, has evolved radically. Originally horizontal timbers fixed to posts, the palisade is now vertical timbers fixed to top and bottom rails attached to posts embedded 600mm in the ground. "A palisade is a defensive structure," says Harrower, "and timbers rising out of the ground with shrubs planted at the base would be harder to attack than horizontal fencing."

On the floors will go spoil from the ditches mixed with water until it's puddled clay – slimy, slippery mud. The site volunteers will then jump around stamping pebbles and gravel into it and finish off by covering it with a few inches of insulating bracken.

Because not all Clanranald volunteers have physiques as impressive as Charlie Allen, health and safety will be the biggest site challenge. Professionals will build the earthworks, but hundreds of volunteers with no experience whatsoever of construction work will erect the timber structures. "Skilled toolhandling will be done by professionals," says Harrower. "It's the manual handling where we want to minimise risk. A lot of people will be going from clerical jobs to lifting A-frames."

Still looking for a construction manager for Duncarron – a part-time job for 18 months if you're interested – Harrower says the successful applicant will have experience of a large site and organising lots of labour, perhaps from running a youth opportunities programme.