The structure's walls consist of three components: an outer layer (tilted inwards and made of many layers of stones whose size diminishes with increasing height: mostly, lower layers consist of rubble masonry, while upper layers tend to be of ashlar masonry); an inner layer, made of smaller stones (to form a corbelled dome of the bullet-shaped "tholos" type, and where ashlar masonry is used more frequently); and an intermediate layer of very small pieces and dirt, which makes the whole construction very sturdy: it stands only by virtue of the weight of its stones, which may each amount to several tons. Some nuraghes are about 20 meters (60 ft) in height, the tallest one known, Nuraghe Arrubiu, reached a height of 25–30 meters.