in the 6th millennium BC by the Cardial culture coming from the Italian Peninsula. In the mid-Neolithic period, the Ozieri culture, probably of Aegean origin, flourished on the island spreading the hypogeum tombs known as domus de Janas, while the Arzachena culture of Gallura built the first megaliths: circular tombs. In the early 3rd millennium BC, the metallurgy of copper and silver began to develop. During the late Chalcolithic the so-called Beaker culture, coming from various parts of Continental Europe, appeared in Sardinia. These new people predominantly settled on the west coast, where the majority of the sites attributed to them had been found. The Beaker culture was followed in the early Bronze Age by the Bonnanaro culture which showed both reminiscences of the Beaker and influences by the Polada culture. As time passed the different Sardinian populations appear to have become united in customs, yet remained politically divided into various small, tribal groupings, at times banding together against invading forces from the sea, and at others waging war against each other. Habitations consisted of round thatched stone huts. Nuragic civilization From about 1500 BC onwards, villages were built around a kind of round tower-fortress called nuraghe (usually pluralized as nuraghes in English and as in Italian). These towers were often reinforced and enlarged with battlements. Tribal boundaries were guarded by smaller lookout Nuraghes erected on strategic hills commanding a view of other territories. Today, some 7,000 Nuraghes dot the Sardinian landscape. While initially these Nuraghes had a relatively simple structure, with time they became extremely complex and monumental (see for example the Nuraghe Santu Antine, Su Nuraxi, or Nuraghe Arrubiu). The scale, complexity and territorial spread of these buildings attest to the level of wealth accumulated by the Nuragic Sardinians, their advances in technology and the complexity of their society, which was able to coordinate large numbers of people with different roles for the purpose of building the monumental Nuraghes. The Nuraghes are not the only Nuragic buildings that stand in place, as there are several sacred wells around Sardinia and other buildings with religious purposes such as the Giants' grave (monumental collective tombs) and collections of religious buildings that probably served as destinations for pilgrimage and mass religious rites (e.g. Su Romanzesu near Bitti). At the time, Sardinia was at the centre of several commercial routes and it was an important provider of raw materials such as copper