Historically, they were a Central European civilization who dominated much of Europe, such as Southern Germany, Spain, France, and the British Isles. The Celts in the game are based on the Celts of Scotland, Ireland, and Wales in Western Europe, but they only speak Irish Gaelic. Their unique unit is the Woad Raider, a type of infantry who covers himself in paint made from the woad plant to make himself look more fearsome. Woad Raiders move very fast, allowing them to overcome the normal infantry weaknesses against archers and siege weapons. Their Imperial Age unique technology is Furor Celtica, which refers to the Celtic warrior tradition of channeling their emotions and anger and makes their siege weapons harder to kill. Being masters of guerilla warfare, Celtic infantry move faster. Because of their historical wood and metalworking, their Siege Workshops build faster, their Lumberjacks work faster, and their siege weapons fire faster. Like their neighbors the Britons, the Celts were historically skilled shepherds, and it is much easier for them to steal their enemies' Sheep.
Their descendants are the Scottish, Welsh, and Irish nations. These three nations, alongside the English nation (descendants of the Britons), created a united kingdom: the British civilization that is featured in Age of Empires III.
The Celts have very good infantry units with a major advantage in a faster movement speed. The other two main branches, however, namely archers and cavalry, are very underwhelming, missing key technologies all over the board. Their use in later stages of the game is usually only considered in special circumstances. Their navy is rather weak as well as they lack Fast Fire Ships and Bracer which puts their Galleons at a disadvantage. Their Monks are among the weakest of all, missing five of ten technologies. The defenses are average and so is their economy with two missing upgrades each. Their Siege Workshop, however, is beyond any doubt. The siege weapons they build are attacking faster, living longer due to Furor Celtica, and built faster, making them extremely dangerous.
All in all, the Celts have a very lacking technology tree, but if they play to their strengths, their army of infantry and siege weapons can easily be enough to defeat any army.
When playing a random map game against the computer, the player may encounter any of the following Celt AI characters:
Aedan: Áedán mac Gabráin, King of Dál Riata (West-Scotland) from 574-609. Held campaigns in Ireland, Scotland and to the Orkney islands, but finally decisively defeated by Aethelfrith.
Aethelfrith: King of Bernicia (East-Scotland) from 593-616. Notable for his successes against the Britons and the Gaels (Aedan), laying the foundation for the future kingdom of Northumbria. Killed in battle.
Ainmire: Ainmuire mac Sétnai, High King of Ireland from 566-569. First king from northern Ireland, but soon killed by a rival prince.
Ambrosius: Romano-British war leader from 479 and onwards, victorious in important battles against the Anglo-Saxons in the 5th century. Could, historically, have been the inspiration of, or have influenced, the King Arthur epic.
Athelred the Unready (966-1016): King of the English from 978-1016. Had to defend England against Danish Vikings (Sweyn Forkbeard) but failed and fled to Normandy in 1013. Died in a campaign to retake England (1016).
Brian Boru (941-1014): High King of Ireland (1002-1014). Fought many battles to unify Ireland under him. His Irish won the final battle against the Norse Vikings of Dublin, but Brian died in battle.
Brude: Bridei I, king of the Picts in Scotland from 554-584.
Columba (521-597): Irish abbot and missionary who spread Christianity throughout Ireland and Scotland. His work would be the basis of Celtic Christianity. Today revered as an Irish Saint.
Conall: Conall Cearnach, a hero in Irish mythology and sagas. His epic led him through Ireland, England and mainland Europe, completing various quests along the way.
Cunedda: Celtic leader, originating from northern England, lived around the beginning of the 5th century. Waged war on the Picts and Irish, and later drove them from Wales where he laid the foundation for the kingdom of Gwynedd.
Diarmait: High King of Ireland from 560-565.
Macbeth (1005-1057): King of Scotland (1040-1057). Defended Scotland from English attacks, died in battle by a rival. Became a legend in Shakespeare’s tragedy Macbeth.
Maelgwn: Maelgwn Gwynedd, king of Gwynedd (northern Wales) in the beginning of the 6th century. Supporter of Christianity, died from the Plague and passed into Welsh legend.
Robert the Bruce (1274-1329): King of Scotland from 1306-1329. Famous warrior who led the Scots in the First Scottish Independence War, resulting in an independent Scotland.
Vortigern: Supposedly King of the Britons, 5th century warlord who fought against the Picts and the Scots.
William Wallace (1270-1305): One of the main leaders of the First Scottish Independence War. Famous for his victory in the Battle of Stirling Bridge (1297). Defeated at the Battle of Falkirk (1298), after which Robert the Bruce took over command of the Scots. William was eventually captured by the English and executed.