The origin of brochs is a subject of continuing research. Sixty years ago most archaeologists believed that brochs were built by an influx of immigrants who had been displaced and pushed northward first by the intrusions of Belgic tribes into south-east England towards the end of the second century BC and later by the Roman invasion of southern England from AD 43 onwards. Yet there was never any doubt that the hollow-walled broch tower was a purely Scottish invention, or that even the kinds of pottery found within them that most resembled south English styles were local hybrid forms. The first of the modern review articles on the subject (MacKie 1965) did not, as is commonly believed, advocate the view that brochs were built by immigrants but rather that a hybrid culture of a small number of immigrants and the native population of the Hebrides produced them in the first century BC from earlier, simpler promontory forts. This view contrasts for example with that of Sir Lindsay Scott who argued—following Childe (1935)—for a wholesale migration into Atlantic Scotland of people from south-west England.
Even this 1965 theory has fallen from favour, mainly because in the 1970s there was a general move away from 'diffusionist' explanations in archaeology towards those involving indigenous development. However, the increasing number of radiocarbon dates for the primary use of brochs (as opposed to their later, secondary use) still suggest that most of the towers were built in the 1st centuries BC and AD. A few such dates may be earlier, notably Old Scatness Broch in Shetland, where a sheep bone dating to 390–200 BC has been reported The other broch claimed to be substantially older than the 1st century BC is Crosskirk in Caithness but a new review of the evidence suggests that it cannot plausibly be assigned a date earlier than the 1st centuries BC/AD
Although mainly concentrated in the northern Highlands and the Islands, some examples occur in the borders (for example Edin's Hall Broch), on the west coast of Dumfries and Galloway and near Stirling. This small group of southern brochs has never been satisfactorily explained.
From the 1930s to the 1960s, archaeologists like V. Gordon Childe, Sir Lindsay Scott and John Hamilton regarded them as castles where local landowners held sway over a subject population.
These theories fell from favour among Scottish archaeologists in the 1980s, due to a lack of supporting archaeological evidence. These archaeologists suggested that defensibility was never a major concern in the siting of a broch, and have argued that they may have been the "stately homes" of their time, objects of prestige and very visible demonstrations of superiority for important families (Armit 2003). Once again, there is a lack of archaeological proof for this reconstruction.
Brochs' close groupings and sheer numbers in many areas may indeed suggest that they had a primarily defensive or even offensive function. Some of them were sited beside precipitous cliffs and were protected by large ramparts: a good example is at Burland near Gulberwick in Shetland. Often they are at key strategic points. In Shetland they cluster round narrow stretches of water: the broch of Mousa, for instance, is directly opposite another at Burraland in Sandwick. In Orkney there are about a dozen on the facing shores of Eynhallow Sound , and many at the exits and entrances of the great harbour of Scapa Flow. In Sutherland quite a few are placed along the sides and at the mouths of deep valleys. Writing in 1956 John Stewart suggested that brochs were forts put up by a military society to scan and protect the countryside and seas.
The key difference seems to be between those who regard brochs as an architectural puzzle, and those who consider them as a social-historical problem.
Some archaeologists are now inclined to consider broch sites individually, doubting that there ever was a single common purpose for which every broch was constructed. There are differences between the various areas in which brochs are found, with regard to position, dimensions and likely status. For example, the broch 'villages' which occur at a few places in Orkney have no parallel in the Western Isles.
On the islands of Orkney and Shetland there are very few cells at ground floor. However, most brochs have scarcements (ledges) which would have allowed the construction of a very sturdy wooden first floor (first spotted by the antiquary George Low in Shetland in 1774), and excavations at Loch na Berie on the Isle of Lewis show signs of a further, second floor (eg stairs on the first floor, which head upwards). Some brochs such as Dun Dornaigill and Culswick in Shetland have unusual triangular lintels above the entrance door.
As in the case of Old Scatness in Shetland (near Jarlshof and Burroughston on Shapinsay, brochs were sometimes located close to arable land and a source of water (some have wells or natural springs rising within their central space). Sometimes, on the other hand, they were sited in wilderness areas (e.g. Levenwick and Culswick in Shetland, Castle Cole in Sutherland). Brochs are often built beside the sea; sometimes they are on islands in lochs (e.g. Clickimin in Shetland).
About 20 Orcadian broch sites include small settlements of stone buildings surrounding the main tower. Examples include Howe, near Stromness, Gurness Broch in the north west of Mainland, Orkney, Midhowe on Rousay and Lingro near Kirkwall. There are "broch village" sites in Caithness, but elsewhere they are unknown.
Most brochs are unexcavated but many of those that have been properly examined show that they continued in use for many centuries, although the interiors were often modified and changed, and they underwent many phases of habitation and abandonment. The end of the broch period par excellence seems to have come around AD 200-300.
The skills involved in broch building are currently being explored by drystone dyker Irwin Campbell and AOC Archaeology Ltd., based in Edinburgh.