A roundhouse is a type of structure built by creating a circular ring of wooden posts that hold up a thatched roof. The walls would likely have been made from turf with potential for wattle screens or partitions inside. Internal features such as pits, and ring ditches are often present and indicate areas where people or animals have been active and worn down the ground. After the collapse or abandonment of a roundhouse, the walls and roof decay and are often removed along with the upper floor layers by later agricultural ploughing so that only the deepest features such as postholes, pits, and ditches survive.
The roundhouse structures found at Craggan are typical examples of Iron Age roundhouses, measuring about 6m across, and often identified by the presence of a post ring and/or larger segments of ring ditches. A metal working furnace also confirms that metal working was taking place on site in the Middle Iron age settlement, most likely for production/repair of everyday tools and domestic items.