Neolithic settlements from around 3,000 BC have been found. Celtic clans settled in fertile valleys in parts of present-day Switzerland from around 1,500 BC, with the Raetians in the east and the Helvetti to the west. A Bronze Age Celtic civilization reached its height around 100 BC. An attempt by the Helvetti to spread west into Gaul was quashed by Julius Caesar in 58 BC. As the Roman Empire expanded northward and westward, Switzerland came under its domain, centred on Aventicum (Avenches). The Romans constructed a road network from the strategically important Alpine passes but attempts to conquer Germanic tribes to the north and east of the Rhine were thwarted in AD 9. Garrisons along the Rhine from Lake Constance to Basle were maintained until Roman forces withdrew in 401.