Although Roman historians have provided commentary on the Celtic tribes current at the time of the Roman invasions in 51 BCE and 43 CE, the archaeological evidence suggests the British Celts, an Iron Age people, migrated from central Europe, c. 450 BCE. Pre-Roman conquest, the Celts lived in hillforts or fortified groups of towns (oppidums), which developed in the first millennium BCE. It is thought that these were constructed as a response to tribal tensions, generated by population increase and pressure on agriculture. The British Celts spoke the Common Brittonic language, usually had one ruler and used coinage, created through iron-working. They had Druidic rites and practised human sacrifice. Tribes, such as the Caledonii, were described by Tacitus as red-haired and large-limbed, while the Silures were a mountain tribe who were war-like, swarthy and curly haired. Some tribes had regional centres, with the ‘market place of the Belgae’ becoming Roman Winchester.