larger fortified settlements expanded, such as the votadini stronghold of traprain law, east lothian, which was the size of a town. huge numbers of small duns, hill forts and ring forts were built on any suitable crag or hillock. the spectacular brochs were built, most impressively the nearly complete mousa broch, shetland. many souterrain underground passageways were constructed, though their purpose is obscure. island settlements linked with land by a causeway, the crannogs, became common; it is thought that their function was defensive.