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Only in Scotland
The word 'broch' comes from the Old Norse 'borg', meaning a fortification. And in the wild, stony landscape of north-west Scotland, during the Scottish Iron Age (from about 700 BC to 400 AD), hundreds of these monumental dry stone towers came to dominate their surroundings. Particularly common on Orkney (where more than 120 had been built or were under construction by 100 BC), Shetland and the Western Isles, at least 700 brochs have been identified altogether across Scotland.
Brochs are not found anywhere other than Scotland. They represent the highest level of achievement in dry-stone construction techniques, reaching a height of up to 13 metres in some of the best-preserved examples. The towers are made up of two concentric circular walls. These walls, which can be up to five metres wide at their base, are separated by a passageway with stairs to different levels.
At the heart of the structure, in the inner circle, was the hearth and the main living areas. There would have been wooden floors at several different levels. The ground floor may have been used as a place of refuge for cattle and sheep at times of danger or bad weather.
It is thought that brochs had conical, thatched roofs. Certainly, they would have towered over the Iron-Age landscape – a statement of strength and power to whoever looked on them. For reasons that are not yet understood, their construction ceased around 100 AD. The fact that so many have survived the past 2,000 years so well is a tribute to the organisation and skills of their builders.
No mortar involved
Ian Armit, a Scottish Iron-Age specialist who worked with Time Team on its excavation of a broch at Applecross, described them for the programme: 'Basically, they are dry stone towers – there's no mortar or anything involved. It's a big stone roundhouse, with a massively thick wall foundation. The best-preserved ones today are up to 13 metres in height. Not every one would have been that high, but it was certainly common for them to get up to that sort of level.
'One of the keys to the construction is that you have this trick of building two concentric circular walls with a corridor or gallery that runs in between them. The two walls lean in on each other. The intricacy of the bonding of the dry stone is what prevents the structure from collapsing. It's a fantastic level of achievement of dry stone masonry to build this. The whole thing holds itself together.'
Double walls
Time Team graphic designer Raysan Al-Kubaisi, who has a background in architecture, pointed out how the double-walled brochs were perfectly designed to withstand the worst of the Scottish weather (which was particularly bad, even in June, during Time Team's Applecross dig). 'The double skin is a response to thermals, how heat is used in the building. If you have a central fire, the hot air rises and you get a convection current, drawing the cold air in through the cavity between the two concentric walls and giving it time to heat up before it enters the living space.'
As well as keeping the occupants of a broch warm, the double walls would also have ensured that rain would not be able to penetrate to the rooms in the inner circle. Even if it made it through the outer wall, there would then be a gap preventing any water getting through to the inner wall.
Deterrent and status
Of course, in addition to keeping out the elements, the broch structure is also perfectly suited to keeping out attackers. There is little evidence of Iron-Age warfare, or brochs having been subject to serious attacks, however, so either their mere presence served as a deterrent to possible enemies or the structures had as much to do with the status of their occupants as any actual defensive purpose.
Archaeologists disagree on the precise functions of brochs, but it is likely that they fulfilled a range of different functions at different times – mostly peaceful (as houses for tribal chiefs or wealthy farmers), but sometimes serving as strongholds at times of conflict.
What no one disagrees on is that the brochs of north-west Scotland are an outstanding engineering achievement by a people about whom we still have much to discover.