Creating a Pictish world...

Once they learnt the tools of their trade, the cast were ready to inhabit the world the set designers had created for them. Simon Bowles and Marshall have worked together a number of times including on Marshall's breakthrough features film Dog Soldiers. Bowles and his team constructed a huge fort complex and Pict village in the forests of Surrey as well as small but beautifully forest dwelling for Imogen Poots' character Arianne.

Bowles had spent many month researching historical data on early settlers in the Caledonian regions of Britain, comparing archaeological finds with other historical writings and artistic depictions. Marshall wanted to stay true to historical records of the Roman Empire and the Pictish people but push the element of drama with what Bowles describes as "bucket loads of mud, piss and blood."

"I immersed myself in all things Roma including visiting sites and museums to look at existing remnants of buildings and objects, such as armour, horse tack, cooking equipment, pottery and etchings from the period showing the way soldiers lived day-to-day," says Bowles of his initial research. Bowles met with re-enactment groups who had previously taken these references and built own equipment. Bowles took on board many of the comments the members gave from their knowledge of this historical period.

"As the Roman period has been the subject of sustained public interest for the last 100 years there are many books covering the subject, again with conflicting information and opinions but all great resources," Bowles explains. "This conflict of opinion actually freed me to create my own look. My design was based on fact and reflected the heightened drama that Neil was looking for, whilst at the same time matching the demands of a tight timescale."

Ideally, for Bowles, the materials they would have used would have come from the surrounding landscape and been built using period tools, but the unforgiving weather conditions meant Bowles had to use come creative licence.

"There is very little record of the Pictish way of life, mainly because they used organic materials such as timber, wool and leather," Bowles explains having spent many month researching the period. "What they did leave behind is a wealth of decorated carved stone, bone and metalwork depicting the animals used in their spiritual culture. I used these symbols on the twelve-foot high standing stone at the centre of the village with the same designs being used by the make-up department for tattoos on some of the male and female Picts warriors. Beyond primary historical sources, I drew inspiration from visits to recreated roundhouse villages in Britain."

"As we were filming over a bitterly cold winter there was a concern the wattle and daub of the round walls not dry so we used hardening spray foam that I first obtained to create the cave walls in Neil's earlier The Descent. This foam was used to manufacture pre-cast walls from a section of real wattle and daub and painted. One advantage of the foam walls was discovered while filming – a section could easily be cut out with a sharp knife to allow the camera access for wider shots inside the houses, then replaced without too much trouble. The foam was fire proof, but the straw used for the roofs was not, so they had to be sprayed with a fireproofing chemical prior to filming."

The script required a Roman fort a day's travel north of Carlisle. This would have originally been a timber fort built from the trees felled to make the clearing in which it stood. "I wanted ot to look as though it had been built by a very regimented group of soldiers," says Bowles. "Men who would have built this structure many times before in many other countries they had invaded."

The 100-foot square for housed leather tent, stable space for horses and livestock, a coking area and a blacksmith's forge for making and maintaining weaponry. Bowles and his team found a large fresh clearing in woodland outside London as starting point. "Using real twenty-foot long trees that would have to be set into the ground by aten feet using excavators was too complicated to deliver in time but I found a compromise in using a mix of real wood and fake wood, again using the foam to cast trees. This turned out to be really fast and safe, meaning we could build three 100-foot sides, creating a really impressive and imposing fort."

"It always helps having a great set," says JJ Field who plays Thax. "Nothing's better than the real thing, instead of the green screen. You just dive into the surroundings that Neil and Simon have created."

Marshall's admiration of Bowles'team's consideration of time constraints is apparent when he refers to his skill and creativity as a production designer. "What Simon does so brilliantly is give me exactly what I need to get the job done. I didn't need to have a complete fort because I was never going to look in one direction anyway. So we made do with three quarters of a fort and used the money elsewhere. And actually it made it much easier because it enable us to get the camera crane and stuff like that in. He finds out what I need; how I'm going to film the sequence; anything spare can be taken over to another set or another piece of art design."

To fully realize the hostile world of ancient Caledonia, similar attention to detail was applied to the clothes worn, the language spoken and how the cast appeared – right down to the colour of their teeth. "Oh yeah. Yellow teeth. Dirty teeth. That's marvelous. We love it," laughs Kurilenko. "You walk on the set and everybody's dressed in amazing costumes. Keith Madden, the costume designer, has done just an incredible job. Really beautiful."

As with the plot, Marshall took his lead history. Then he filled in the gaps with a mixture of imagination and logical conclusions. Although Marshall admits there is very little historical reference to the Picts, there were certainly signs from history that they were a sophisticated community with a number of advanced skills such as carpentry, metalwork, weaving and farming.

As there was also no written documentation of their language, Marshall took certain artistic licence and supplanted English dialogue with Scots Gaelic. "I thought, OK, that's going to be the closest that can possibly get to what they might have spoken," says Marshall.

Time and again, Marshall tested the internal consistency of the world he'd created. He knew the image of Celtic Britons running into battle, their skin emblazoned with tattoos and blue woad was one he'd envisioned when he wrote the script. "I thought, what are the chances of them running into battle on a freezing cold winter's day wearing nothing but a little kind of loin cloth of whatever? It seemed illogical to me. Maybe they do that in the height summer but in the winter they are going to be pretty much like everybody else and want to wear warm clothes. So, you know, they have their tattoos and war paint. But we've dressed them in furs that would have been readily available to them in the Highlands – deerskin, bearskin."

He applied a similar logical progression to the film's large-scale sequences. The inspiration for the ambush of the Ninth Legion was guided by the natural hilly landscape. Marshall questioned how they would have used the Highlands to their advantage. It is well documented the Romans were a superior fighting force. Marshall concedes "if you got them out the open there was just that they were going to lose."

Marshall dipped into Roman battles from some of the great Hollywood stories to find inspiration. "I looked at Spartacus with the rolling logs," he says. "I though, 'well, we couldn't do rolling logs because we're in a forest. So what about big fireballs that would roll down, smash the Roman lines apart, and create the advantage? They could then send their shock troops down and just smash the Romans to pieces.' Most of that wasn't really taken from a typical tactic used by the Picts. It was just me thinking, if I was a Pict, how would I beat the Romans here?"

He credits the on-set special effects team with creating a realistic battlefield. "The special effects team did an amazing job with the fireballs. They used these ramps with levers on them. They put a ball on there, soaked it in paraffin, set fire to it and pulled the lever and it came storming down the hill and smashed into these stunt guys at the bottom. With four or five fireballs coming down at the same time, it was pretty spectacular."

Michael Fassbender looked on with similar admiration for the technical crew. "It was impressive to see every element of art work – all these components really just coming together to make this finished product."

While Marshall strove to create a world in-camera using location, sets and specially trained actors, there was always going to be a digital consideration and the use of advanced computer-generated special effects to expand that world.

Rather than rely on recruiting several thousand extras to emulate the size and strength of the Roman Legion Marshall was able to use advanced in digital to create the matching masses. "We only needed 100 or so men. With the digital visual effects in mind, we'd get them to march up the same road ten times, put it all in the computer, and watch it come out as though you have a legion. It stretches on as the eye can see and, in that way, we got the scale we wanted."