Aosta was settled in proto-historic times and later became a centre of the Salassi, many of whom were killed or sold into slavery by the Romans in 25 BC. The campaign was led by Terentius Varro, who then founded the Roman colony of "Augusta Praetoria Salassorum", housing 3,000 retired veterans. After 11 BC Aosta became the capital of the Alpes Graies ("Grey Alps") province of the Empire. Its position at the confluence of two rivers, at the end of the Great and the Little St Bernard Pass, gave it considerable military importance, and its layout was that of a Roman military camp.