Although brochs have been around since the bronze age, most surviving Highland brochs were built between 100 BC and 100 AD, a period of just 200 years that coincided with the arrival of the Romans, who first landed in England in 55 BC. By 47 AD, the Romans had conquered the whole of the south of England and declared Britain part of the Roman Empire. It was during this time that the Picts were hurriedly constructing brochs all over the Highlands, so there can be no doubt that they were built with military defensive purpose.
By 79 AD, the whole of England had been conquered and Agricola attempted to conquer Scotland. After a number of failed military campaigns, which included the annihilation of the 9th Legion around 117 AD, the Romans retreated south and built Hadrian's wall for their own protection.
In 208 AD, the Romans marched again to conquer Scotland. In 212 AD, they again left defeated. In 367 AD, the Picts with the help of the Irish invaded England and together they pushed the Romans back from their last defensive positions at Hadrian's wall. Not long after that, the Romans left Britain. It is plausible that the defeats suffered by the Romans at the hands of the Picts were instrumental in laying the groundwork for the collapse of the Roman empire.
As practically all Highland brochs were built during the time of the Roman invasion of Britain, surely they must have been built with military purpose. The failed Roman military campaigns must be directly attributable to the defensive qualities of brochs against swords, bows, and spears. Roman legions, after all, would hardly have been hauling siege engines up remote Scottish glens.
The battle of Mons Graupius in 83 AD is in all probability a Roman myth, fabricated to placate the Senate in Rome, who would have found it difficult to believe their legions could not take Scotland. If the Romans had indeed conquered the Scots and routed them at Mons Graupius, why then did they not take the country, but instead retreated south not many years later and cowered behind their wall? And what of the 9th Legion, which simply disappeared while on operations in Scotland? That a 9th Legion officer later showed up in another part of the world isn't proof the 9th wasn't destroyed, it is merely proof that one of their officers ran away and escaped.
According to Tacitus, the Roman Historian, the battle of Mons Graupius was a decisive Roman victory in which the Caledonii army was destroyed and scattered. According to Tacitus, over 10,000 Caledonii were killed in battle for the loss of only 360 Romans. That's what Tacitus claims. Let's look at the facts. While Agricola was advancing north into Scotland he consolidated his gains with massive fortresses. After the battle of Mons Graupius, Agricola built no fortresses but instead retreated to his established forts further south. That same year, Agricola was recalled to Rome and was later murdered by the Emperor. Two to six years later, the Romans retreated even further south to their fortresses along the Clyde/ Forth isthmus. Not long after that the Romans were pushed out of Scotland and built Hadrian's wall for their own protection. The facts speak for themselves. Warren MacLeod has written a brilliant analysis of The Agricola by Tacitus, and has some compelling evidence pointing to Forres as the site of the battle of Mons Graupius. You can download his analysis of the Agricola in pdf format here (35Mb).
Calach (Calgacus), the Pictish leader who united the tribes of Scotland, is said to have described the Romans as Robbers of the world, having by their universal plunder exhausted the land, they rifle the deep. If the enemy be rich, they are rapacious; if he be poor, they lust for dominion; neither the east nor the west has been able to satisfy them. Alone among men they covet with equal eagerness poverty and riches. To robbery, slaughter, plunder, they give the lying name of empire; they make a desolation and call it peace.
Unfortunately, as so little is known about brochs and the Picts, much of what is known today is merely guesswork and conjecture. However, we can dispel a few of the myths surrounding brochs by simply narrowing our field of view to the period 100 BC to 100 AD and the Roman invasion of Britain.
One such myth is that they were built purely as prestigious homes for a Pictish aristocracy. Ask yourself a question - do people build elaborate and expensive private houses during a savage time of war? No, they don't. The Picts were fighting for their lives and their future was uncertain, so they would hardly have been building brochs as homes for one or two wealthy individuals. Had brochs been built purely as status symbols surely there would have been a steady stream of them being built over many hundreds of years, but this just isn't the case as most of them sprang up over a 200 year period that coincides with the arrival of the Romans. Additionally, private dwellings tend to be built to personal preference, each one unique and individual. They are certainly not all built to one basic standard blueprint, as nearly all brochs are. As many brochs were also built as extensions onto existing stone round houses, which would indeed have been prestigious private dwellings, there can be absolutely no doubt as to the military defensive intent behind their construction.
Were they effective military defences? Some say they had poor military defence capabilities. This might be true against cruise missiles, but against swords, spears, and arrows I'm sure they were more than adequate as defensive forts. Oh, but you could just set fire to them and smoke everyone out, couldn't you? Set fire to what? 10 ft thick double skinned stone walls? Have you ever tried to set fire to 10ft thick stone walls? That's absurd. As to smoking out the Picts, didn't the Romans set fire to Caledonia and burn down all her trees to do just that? It didn't work, did it?
Another theory as to their use was as places of worship, and that the entrances were low to force people to bow as they were entering. I have difficulty with this. Would the Picts have been preoccupied with suddenly building hundreds and hundreds of broch towers all over the highlands so folks could go to church just as the Romans were invading? If indeed they were simply places of worship, would their walls have required 10ft thick double skinned stone and would they have required two or more brochs in some locations?
I read once that someone thought most brochs were low level round houses owing to the fact that there was so little stone lying around, that if such a huge tower collapsed there would be more stone. For a start, most brochs did not collapse and would still be standing in their entirety to this day had their stone not been robbed to build stone dykes, farm buildings, and even modern roads. It is only in the last couple of hundred years that many brochs have been reduced to rubble for their stone.
A remarkable fact regarding brochs is that most of them are in direct line of sight of other brochs. The only possible reason for deliberately building brochs with direct lines of sight has to be communications. You can track the brochs up the Strath of Kildonan for example, from Kilphedir to the Suisgill broch, with Eldrable, Gailiable, Balvalaich, Kilearnan Hill, Kilearnan, Learable, Ach An Fionnfhuraidh, and Carn Nam Buth. The only possible broken link in this chain appears to be around Kildonan, where there could have been a tower at some time linking Learable with Kilearnan, which although are in line of sight are separated by great distance. Also bear in mind that Caledonia was a huge natural forest at that time before the Romans burned it all down, so the tops of brochs would have had to poke above the forest canopy.
If this theory is correct, this chain would have without doubt continued down the Strath to Helmsdale and linked up with a chain of brochs heading north and south along the coast. The brochs at Brora would have linked up with the brochs at Loch Brora, while the brochs at Golspie would have linked with brochs in Dunrobin Glen. There would also have been a chain of brochs heading inland up Strath Fleet, linking the Rogart brochs with the Lairg brochs. As a case in point, the Skelbo Wood broch is sited so as to be in view of both the Dun Robin and East Kinnauld brochs, which would have linked along the coast via Carn Liath and others to Brora and Helmsdale. Cairns and Duns would also have played their part. For example, the Loch Brora broch (Killin), although in view of the brochs on the south shores of Loch Brora, isn't in direct line of sight with Caistel na Coille on the banks of the Black Water or Coich Burn further up Strath Brora. However, there is a chambered cairn near the summit of Balnacoil Hill which is in direct line of sight of all three brochs, making it an integral link in the communications chain. If there are broken links in any broch chains today, this should make finding missing broch, cairn or dun sites a little easier.
Communications of this kind can only point to military strategy. How were communications passed along? Well, the only warnings needed were that of Roman landings, so any prearranged signal would have sufficed. Banners raised on poles from the tops of brochs would have further extended the range of such communications. Good intelligence would have been invaluable in passing word around that Romans had landed so the Picts could organise themselves quickly. I would suggest that perhaps even the entire Highlands of Scotland was interconnected through an elaborate network of brochs, cairns and duns. If brochs were isolated and cut off, without lines of communications to other Pictish settlements, the Romans could have picked them off one by one quite easily. If lines of communications were open and all brochs were interconnected through line of sight, there is no way the Romans could have taken Scotland quietly. If my theory is correct, then within an hour of landing, word could have passed throughout the entire Highlands and the Picts could have mobilized and marched as a united army while those near any Roman landings would have taken refuge in their brochs until help arrived.
As all brochs share basically the same architectural design, were built to specific blueprints and exacting standards, and most of them sprang up over a 200 year period it is most likely that they were built by national construction teams that went around the Highlands fortifying Scotland. There is organisation and feverish industry at a national level here, with architectural and military genius working together.
There is little doubt that the Picts pulled together as a country to defend themselves from the Romans, suggesting effective and efficient national networks of communications, while teams of architects, stonemasons, and labourers built the brochs with military defensive purpose. The Picts were not savages, they were intelligent, they were organised, they were industrious, they were warriors, and they defeated Rome.