The Scottish Iron Age began somewhere between 800 and 500 BC, although very little iron would actually have been used at that time in Scotland. Early iron tools and weapons are likely to have been of variable quality, some probably inferior to their bronze counterparts, achieving superiority only with the ability to control the carbon content and quality of the iron.
The circular hut was still the basic house form, but there was a tendency towards much more substantial walls further thickened at the entrances to form an entrance passage. Some examples incorporate a structure known as a souterrain a long, narrow, curving underground chamber, having a deliberately constricted entrance and widening slightly at the distal end. The souterrain entrance opened into the hut, thus affording control of access.
Souterrains also occur as isolated structures, their purpose is not known. Some authors have suggested they were for the storage and control of food, others have suggested they were places of refuge or had a ritual function.
The building of hill and promontory forts is also seen as a late Bronze to early Iron Age phenomenon. The forts amount to an area of land cut-off by a ditch and bank rampart, that appear, at first sight, to be defensive. This should not be taken necessarily as indicative of warfare. The term fort implies a singular defensive purpose that may not reflect the actual use or uses of these enclosures. The proliferation of richly decorated weaponry at this time might suggest that these structures are an expression of a martial aesthetic style favoured by an elite and indicative of status rather than warlike intent.
Neither can it be assumed that all such monuments were built during the same period or for the same purpose. Enclosed hilltops are also known from the Neolithic period. One such site-type, known as a causewayed enclosure, appears to have had a social or ritual rather than a defensive function.
The gradual elaboration and aggrandisement of domestic architecture, contemporaneous with the building of hill forts, is also a feature of the Scottish Iron Age. From c. 600 BC onwards, we begin to see the appearance of even more substantial roundhouses that exhibit a variety of forms in different areas of Scotland that fall into the catchall term Atlantic roundhouses. This classification of monuments includes increasingly elaborate complex roundhouses and the ultimate development of the form, the broch which reached its height between 100 BC and 200 AD.
There are reportedly 110 definite and 87 possible brochs in Caithness. Some brochs have substantial peripheral settlements that remained in occupation or were re-occupied after the broch had gone out of use. The houses in these peripheral settlements are often cellular in form, sometimes referred to as cloverleaf or figure-of-eight houses. This house form marks a departure from the circular plan and is seen as late Iron Age or Pictish. They were probably still in use at the time of the Viking incursions of the ninth century AD. However some of these extramural broch settlements are thought to be contemporary with the primary broch and constituted a so-called broch village. This particular form is more common in Orkney and Caithness than in the Atlantic west.
It has been argued that the thick walled Atlantic roundhouse of West Coast Scotland is the common house form of the typical Iron Age farming family, having no connotations of high status. In contrast the broch villages of Caithness and Orkney may be the demesnes of local chieftains and their vassals reflecting a regional variation in Iron Age culture. It is conceivable that the differences between Iron Age societies of the west and those of the northeast may, in part, reflect differing influences of Roman Britain either as a perceived military threat or acculturation through trading contacts. Roman pottery and is known from a number of broch sites in Caithness and Orkney.
A sub-rectangular variant of these post-broch buildings, called a Wag, appears to be particularly associated with Caithness. The term Wag is a corruption of a Gaelic word meaning a small cave, and is descriptive of the semi-subterranean character of the building. Wags are characterised by having two rows of orthostats forming a central aisle. The orthostats support large flat slabs that extend inwards from both sidewalls. A third row of flat slabs covers the central isle to form the essentially flat roof of the building. It is not certain that these were built as dwellings; they may have been built for housing animals.
The remains of houses and fields dominate the visible archaeology of the Iron Age society. This stands in contrast to the Neolithic where the visible archaeology is predominantly funerary or ritual with little evidence of their houses or fields. While Iron Age burials are not unknown, they are comparatively uncommon. Cremation and the scattering of ashes or the placing of bodies in rivers have been suggested as plausible explanations.
One might think that the lack of discrete Iron Age ritual monuments is indicative of decline in the importance of belief systems in Iron Age society. Such a conclusion would be unsafe however. The ritual monuments of the late Neolithic arguably relate to the heavens and may have been physically and perhaps culturally separated from domestic life.
In contrast, Iron Age mythology appears to have focused on subterranean or water dwelling (Chthonic) deities and to have been completely integrated with domesticity. Some roundhouses appear to have been deliberately built into chambered cairns, using the burial chambers for ritual purposes within, rather than separate from the home. The deliberate deposition of human and animal body parts within the walls and under the floors of dwellings is also known and further strengthens the view that the secular and sacred were indivisible in Iron Age Scotland.