The brochs are the result of a long tradition of small fortified farms built from 500 BCE. The Broch of Mousa, which dates back to the first or second century of the common era, makes quite an impression when you are inside, with its shape that is wider at the base (13m high, 15m diameter at the base). The inner courtyard and central hearth were surrounded by wooden lean-tos. Three doors lead to cells within the walls, and a fourth to a staircase up to the top.