The most ancient traces of human presence in Aosta Valley date back to Upper Neolithic but it's only by III Millennium that we have a long-lasting occupation of the valley. In fact some important relics are from the time and it's also when megalithic architecture starts to spread through.
It was the land of Salassi. They settled here in the Iron Age and they had been controlling the valley for a long time with road-taxes. They also did banditry and worked metals. Salassi caused Romans' reaction getting against neighbour tribes to have the total control of Dora river. After having beat and conquered the Salassi, Romans found Augusta Pretoria (now Aosta) to get control of the land and its roads. The area become part of Regio XI and under Roman Empire is at its height.
There still are visible few monumental architectures as bridges and aqueducts of the time. However, the first barbaric invasions start the decline of Roman Aosta and the decay in the whole area.