One summer evening in 335 BCE,  Alexander the Great was resting by the Danube River after  a day of fighting the Scythian tribes when a band of strangers  approached his camp. Alexander had never seen anything  like these tall, fierce-looking warriors with huge  golden neck rings and colorful cloaks— so he invited them to feast with him. They proudly said they were Keltoi  or Celts who came from the far-away Alps. Alexander asked what they feared  the most in the world, hoping they would say him. They laughed and said they feared  nothing at all. This is one of the earliest stories  about the ancient Celts. While we don’t know  where the first Celts came from, by Alexander’s time  they had spread across Europe from Asia Minor in the east to Spain and the Atlantic islands of Britain  and Ireland in the west. The Celts were never one unified empire, and they didn’t build cities or monuments. Instead, they were hundreds of independent tribes who spoke the same language. Each had its own warrior-king  and religious center. The tribes fought each other as enthusiastically as they fought  their enemies. Few armies could stand up to them. Somewhat unusually for the time,  the Celts believed in reincarnation— that they would be reborn on Earth  to live and feast and fight again, which may have contributed  to their fearlessness in battle. Some of them fought naked,  scoffing at their enemies’ armor. The greatest trophy a Celtic warrior could possess was the severed head of a foe. They preserved these heads  in jars of cedar oil and showed them to guests  who visited their homes. Celtic warriors were so valued  in the ancient world that foreign kings often hired  them as mercenary soldiers to serve in their armies. But the Celts were much more  than just warriors. Among them were many skilled craftsmen, artists, and great poets called bards. The bards sang of the brave deeds  of their ancestors and praised the accomplishments of warrior kings— and composed biting satires  about cowardly or selfish leaders. The Celts worshipped many gods, and priests known as druids  oversaw this worship. Anyone could become a druid, but the training required many years  of study and memorization— the druids were not allowed to record any of their teachings in writing. Druids supervised religious practices  and sacrifices to the gods, but they were also teachers, healers,  judges, and scientists. They were so respected that they could step between warring tribes in the middle of a battle  and call an end to the fighting. No Celt would dare to harm a druid, or question their decisions. In the 2nd century BCE, the Romans  began to encroach on Celtic territory, conquering the tribes of northern Italy. Rather than unite against the Roman  legions in response to this defeat, the Celts maintained  their tribal divisions. The tribes of Spain fell soon after. In the 1st century BCE, Julius Caesar  marched his armies across France, using bribery, threats, and lies  to turn tribes against each other. Only in the closing days of this great war did the Celts unite  against their common enemy under the leadership  of king Vercingetorix, but it was too late. Countless warriors and their families  died or were enslaved as the Romans conquered France. Protected by the surrounding waters, the Celtic tribes of Britain and Ireland were the last holdouts. When the Romans finally invaded Britain, the queen Boudicca united her tribe  in a revolt after her husband was killed. She almost succeeded in driving  the Roman legions out of Britain before dying as she led a final battle  against the enemy. By the end of the 1st century CE,  Ireland alone, far out at sea, remained unconquered by Rome. There, the ways of the ancient Celts  survived untouched by the outside world long after Rome itself lay in ruins.