Iatmul villages, containing 300-1,000 people, are built high on riverbanks. Villages often consist of three distinct sections, with a men's house in the center. Houses were often built in two rows, parallel to or at a right angle to the course of the river. The men's house was usually built in the center of an open space, the dancing ground. Older Iatmul men's houses, which were huge buildings up to 20 meters high and 40 meters long, are among the most impressive architectural achievements in New Guinea. They served as men's assembly houses in daily life and as religious centers during rituals. The dancing ground contained a ceremonial mound on which heads were displayed when brought back from a successful raid. Each section of the tripartite village owned a long war canoe that was a symbol of its cooperation during warfare, as was the ceremonial house for ritual life. The whole village usually constituted a defensive unit, whereas only a section of it may have made a raid on an enemy village. A village often was surrounded by fences and watchtowers. Traditionally, Iatmul houses were huge pile dwellings with the families of brothers living together in one house. Clans are classified into moieties, a fact that can be recognized in the layout of the village and the distribution of the houses there.