Huge boulders create an impenetrable 100 metre long and 3 metre high wall. It’s the great wall of the fortress built by pre-Nuragic settlers to defend their wide fertile valley, and it is one of the most remarkable and massive settlements from the III millennium BC in all of the Mediterranean basin. It sits on the slopes of Mount Baranta just 3 kilometres from Olmedo, an important industrial/agricultural town in the heart of the Nurra region. Also known as su Casteddu, the fortress dates to the Copper Age (2500-2000 BC) and was constructed with massive megalithic structures following the natural contours of the terrain. Protected by walls, the fortress is comprised of a wall-tower and a group of rectangular huts. Everything here is imposing, built with large rocks and boulders: even the horse-shoe shaped tower is huge, six and a half meters thick and nine metres high. It sits perched on an outcrop of trachyte over the valley below.