Broch of Mousa

The broch, a dry-stone round tower of the Scottish Iron Age, was built around 300 BC on the western shore of the island of Mousa in Shetland, one of more than 500 raised across Scotland. It stands 13.3 metres high, the tallest still standing, and is among the best-preserved prehistoric buildings in Europe.

It has one of the smallest overall diameters of any broch, one of the thickest wall bases and one of the smallest interiors. That massive construction, along with its remote position, is likely the main reason it has survived so well. It is built of dry stone with no mortar, and it is the only broch complete nearly to the top, including the original stair running within the thickness of the wall.

The single ground-level entrance is on the west side, altered at various times from its original form. The entrance passage runs 5 metres and still has its original bar-hole, cut to take the beam that barred the door. Inside, the central space holds a hearth and a stone floor tank, with a low stone bench around the base of the inner wall that belongs to an early alteration.

The broch went through at least two phases of occupation. In its original condition it may have contained a wooden roundhouse - a circular timber dwelling - resting on the scarcement ledges, the internal wall shelves at heights of 2.1 and 3.7 metres, and presumably on a ring of posts set into the primary floor. It has recently been argued instead that those ledges carried scaffolding in a building that was never roofed.

At a later date, if the timber building existed, it was demolished to make way for a small wheelhouse in the interior, a form in which stone piers project inward from the wall like the spokes of a wheel; this one has three. Three large cells are built into the thickness of the ground-floor wall, entered over thresholds 0.7 metres above floor level, with openings above each lintel that seem designed to let light and air into the chamber behind. All three have recesses set into the wall.

Above the solid base are six galleries, formed by the space between the two concentric walls of the upper tower and partly lit by voids. Most can be walked along. They were probably an aid to the builders in raising the structure rather than accommodation or storage. The stair begins at the second level, reached through a doorway in the inner wall face with a cell beside it, and there is a further cell above the entrance passage.

The broch went on being used for centuries and appears in two Norse sagas. Egil's Saga tells of a couple eloping from Norway to Iceland who were shipwrecked and sheltered in it. The Orkneyinga Saga records a siege of the broch by Earl Harald Maddadsson in 1153, after the abduction of his mother, who was held inside.

The antiquarian George Low visited on his tour of 1774 and made the first drawings. Sir Walter Scott came in 1814 and called it a Pictish fortress, the most entire probably in the world. The geologist and antiquarian Samuel Hibbert gave a detailed account after visiting in 1818, and Sir Henry Dryden made the first accurate survey in 1852 and 1866.

The interior was first cleared of accumulated rubble in 1861, more than two thousand years after the tower was built. The clearing produced large quantities of animal bone, particularly otter, the animals having probably lived in the abandoned structure, along with pottery fragments, stone pot lids and a slaty object described as resembling a triangular file. The most unusual find was a carved wooden model of a Norwegian boat, about 0.9 metres long and made of fir.

Further clearing by the Office of Works in 1919 recovered relatively little. Some pottery sherds, including part of a Black-burnished ware vessel, are now in the National Museum of Scotland. Because the tower never collapsed, and because later reuse and nineteenth-century clearing removed what had gathered inside, the archaeology here is thin and the architecture itself carries most of the evidence.

Modern work has concentrated on recording and conservation rather than excavation. Major stabilisation was carried out between 1967 and the 1980s, and in 2005 archaeologists used 3D laser scanning to document the structure and support conservation planning. The site is managed by Historic Environment Scotland as a scheduled monument.

Mousa is well known among birdwatchers for its breeding European storm petrels, best seen after dark on partly or wholly overcast summer nights. The island holds around 6,800 breeding pairs, about 8 per cent of the British population and about 2.6 per cent of the world population, and some of them nest in burrows within the broch itself.