Scottish Iron Age stone tower. These circular structures often stand within an enclosure which contained round or oval huts and probably acted as strongholds, refuges, or possibly strategic fortifications. They exist mainly in northern and northwestern Scotland, and some were still inhabited at the time of the Viking raids around the 9th century AD.
All brochs show the same basic construction, though there may be a difference in detail. On the outside the walls incline inwards, broken only by a doorway opening, while inside they rise vertically around a central courtyard. Tiers of narrow, circling galleries are built into the interior walls, connected by a stairway from base to summit. At ground-floor level these chambers are small and beehive-shaped.
The entrance was defended on one or both sides by a small chamber within the wall, and has holes for a sliding guard-bar. The inner courtyard, which may have been roofed in and often contains a well, is the only source of light and ventilation to the galleries above. Apertures in the gallery walls are placed in perpendicular rows, separated from each other by single slabs of stone which also serve as a structural feature, providing support without a mass of stone.
No broch has been found complete in its upper parts, but the height can be estimated by the diameter and thickness of the walls at the base. The interior diameter of the base varies between 7.5 m/25 ft and 11 m/35 ft, the walls at ground level being about 4.5 m/15 ft thick.
About 500 brochs are known in Caithness, Sutherland, and the islands of Orkney, Shetland, and the Outer Hebrides. The broch on Mousa in the Shetlands is an exceptionally fine example.