The Celts lived in a widespread territory spreading throughout England, France, Scotland, Ireland, and Wales until around 44 B.C. when Julius Caesar got his head stuck in the salad bowl and started attacking Gaul (the Celtic land). I mean, to attack this place you have to have some nerve.
The Celts were insane when it came to fighting, equipped with iron swords, spears, and huge wooden shields big enough to cover even the largest of these huge people, who were usually nudists. Painted like monkeys while playing ear-piercing music, these warriors were a terror on the battlefield. A roman historian named Tacitus wrote that the Celts' infantry was their main strength though their cavalry of chariots was still very fierce.
Polybius, a Greek historian, wrote that the Romans were horrified by the nudity and noise. The instant the Celtic infantry lifted their weapons they charged like rhinos, throwing their body at people sword-first.
The Celtic chariot riders attacked something like this: first the driver would go all around while the rider continually threw weapons at everyone he could, murdering anyone in his path. When surrounded by other cavalry the warrior would jump from the chariot and go on randomly killing people.
One other thing that made the Celts harder to beat is that they believed that no matter what, when they died it would be just like where they were except with no sickness or hunger or a bunch of other bad stuff.
So if you ever time travel, don't get on these guys' nerves.