Before Europe was conquered by Rome, Celts, who were themselves divided into smaller tribes, had become rather advanced in many ways. Their society was divided among three classes: commoners, the educated, and aristocrats. They formed loose federations of tribes, raised crops and livestock, used the Greek alphabet to write their own language, and were among the first peoples in northern Europe to make iron. They also developed a form of metalwork that most people today recognize as Celtic, called La Tène. They never formed one united nation, however, so that when Roman armies swept across Europe, the Celtic tribes were overrun.