The Celts were a prehistoric people whose numerous tribes occupied much of Europe between c.2000 and c.100 BC, the peak of their power being around 500–100 BC. No European Celtic literature survives, but the later Irish and Welsh sources tell much about Celtic society and way of life. Primarily an agricultural people, though in local areas crafts and iron smelting developed, they grouped together in small settlements. Their social unit, based on kinship, was divided into a warrior nobility and a farming class, from the former being recruited the priests or druids, who ranked highest of all. Celtic art mixes stylized heads with abstract designs of scrolls and spirals (but see La Tène. Remnants of Celtic languages are to be found in the forms of Gaelic, Erse, Manx, and Welsh. The Celtic sphere of influence declined during the 1st century BC owing to the simultaneous expansion of the Roman Empire and the incursions of the Germanic races.