Visible from, and signposted from, the fast SS131 that runs vertically down the island. However only accessible when going north - the road bends quite sharply around Macomer and then heads west and uphill - watch out for the sign to the right.
We always seemed to be on the wrong side of the road, or heading home late after a full day, so never did stop to investigate.
Margaret Guido, in "Sardinia: Ancient Peoples and Places" describes the site as "an early tower, with niches, guard-chamber and staircase has a four-towered addition each of whose towers contains a tholos chamber". It's one of her recommended places to visit.