Using Gallic WarriorsEdit
Gallic Warriors have two key advantages. One, they can be built with Copper as well as Iron. Two, that they have access to the Guerrilla line of promotions. Being built with Copper means that you'll be able to produce them earlier than you might normally, without having the strength disadvantage of Jaguars. This makes them decent at a fast attack, but other Swordsmen replacements are better at this role, the Jaguar being built earlier and slightly faster to build, the Roman Praetorian being an absolute city-crushing juggernaut.
The key uses of Gallic Warriors are in defense, and for pillaging. In defense, they match up to Longbowmen for hills defense - your Mines are safe from enemy attack, and if your cities are on hills, you'll be able to keep them better-defended than Archers might. For pillaging, promote them to Guerrilla II and take out enemy mines. High defense will make your Gallic Warrior a pain to kill and the movement bonus will let you out faster if need be.
Against Gallic WarriorsEdit
Gallic Warriors off hills are no more powerful than regular Swordsmen. In more open terrain, Mounted units can pick them off fairly well. If pillaging warriors becomes a pain, just defend those hills with Archers. Then, they'll lose their advantages.
Pick them off with Axemen! Lone Gallic Warriors are still highly vulnerable to Axemen like regular Swordsmen are.
The warriors of the Celtic tribes were widely feared by the inhabitants of the more civilized regions on the northern side of the Mediterranean. The Celts were known as tall, wild barbarians who fought with a savage fury. Conspicuous for their bravery, there are numerous accounts of Celts charging naked into battle with total disregard for their own safety. The Celts would often paint their bodies different colors to increase the intimidation factor. When clothed, the Celtic fighters wore brightly colored shirts and trousers, featuring striped or checkered patterns for decoration. They sometimes wore bronze helmets decorated with horns or wings in order to make themselves look even taller. The Celts were also head hunters, cutting off the heads of their defeated foes to serve as trophies. This was a way for a warrior to win great respect in Celtic society, and it may have served some religious purpose as well. The Romans were justifiably terrified of this practice and would often kill themselves rather than be taken captive by the Celts.
The Celts are believed to have invented mail armor, which the Romans would adopt from them after the Gaulish sack of Rome in 390 BC. In battle, most Celtic warriors were probably equipped with spears, a simple weapon and easy to produce in large numbers. However, the Celts are much more famous for their use of large, two-handed iron swords. The Celts were the first to create these broadswords, which were used for slashing at the enemy (very different from the Roman short sword, which was entirely limited to stabbing). Celtic chieftains or other high-ranking figures were the ones most likely to carry these swords.
The Celtic warriors were individually brave and fearsome fighters; however, they were also famously undisciplined. There was virtually no unit drill, and thus no unit cohesion: Celtic battles were wild melees in which every warrior fought as he thought best. Thus, while unmatched in personal combat, the Celtic warrior fared poorly against the group unity and strict discipline of the Roman legion.
The Celtic tribes had almost no political unity either. Regarding each other with fear and suspicion and constantly engaged in petty internecine squabbles over territory, they were unable to pull together to face the approaching Roman threat. Facing this divided foe, General Julius Caesar was able to conquer Gaul in under a decade.