It once had a Templar function, as can be determined by the discovery within its central chamber of an altar decorated in the shape of Nuraghe, whose reproduction is in the archaeological museum dedicated to the site. The Nuragic complex Su Mulinu rises up on a small hill overlooking the valley of the Rio Mannu, less than a kilometre from Villanovafranca, the heart of eastern Marmilla. The Nuraghe complex is a juxtaposition of various construction types, from the ‘corridor’ to the tholos roof (false dome). The initial layout, consisting of blocks of marlstone, dates back to the 16th to 15th century BC (Middle Bronze I), with a village arising contemporaneously. During this period, a bastion was built with an outwork, where corridors and cells can still be seen. In the 14th century BC, a trilobed fortification and an outwork composed of four towers united by curtain walls were superimposed. On the lower floor were cells and passageway, with a number of rooms on the upper floor (the focus of excavations). In the third phase, the outwork was reinforced with a fifth tower. The upper layer of one of the rooms with a tholos vault has revealed late Punic, Roman Republic and late-Bronze-early Iron finds, with recent Bronze finds in the lower level. Another similar room revealed materials from the early Middle Ages. Below the ‘historic’ levels, a slab floor was found with a hearth. It was here that fragments from the late Bronze to the Iron Age came to light.
Su Mulinu has been a place of worship since the 14th century BC, as is evidenced by two ceremonial hearths in the central room of the fortress. The ceremonies, perhaps suspended for a few centuries, resumed between the end of 11th and 9th centuries BC, a period to which date back bench-seats and the most extraordinary discovery of the complex: an Nuragic altar in sandstone from the early Iron Age (8th century BC). On top is a basin for collecting liquids, with a connected channel to a tank, into which they would be poured. The monument, used for sacrifices and votive offerings, was decorated with four swords (three intact) that held blades in bronze. Above, were decorations of other bronze objects with anthropomorphic and animal figures, perhaps linked to the legend of the growing moon at the base of the mysterious Nuragic cult, which resumed in Roman times, especially between 50 BC and 150 AD. The materials found during the excavations (still underway) are exhibited in the civic archaeological museum Su Mulinu, set up in the 19th-century Ex Monte Granatico in Villanovafranca, together with ceramic, metal, glass and stone finds from the surrounding area, ranging from the 4th millennium BC to the 3rd century AD. The exhibition shows the development of complex Nuragic architecture spanning more than a millennium (16th-6th century BC) and a spectacular reconstruction of the Nuragic altar.