The Celts are a western European civilization in Age of Empires II standing as an infantry civilization. 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 versus archers and siege weapons. Their imperial 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 opponents' 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.
The Celts are an infantry civilization with powerful siege weapons and very good infantry. The Barracks has all infantry units available, except the Eagle line units, and they have access to all Siege Workshop upgrades, except Bombard Cannon. Their siege units are some of the best in the game, with the fastest creation speed, the greatest HP and fastest fire rate. They also possess the fastest infantry in the game, allowing their Spearman line to be a much greater threat to Knights, and allows their other infantry to stage fast raids on settlements. The Celts' unique unit is the Woad Raider, which is the fastest infantry in the game and costs little gold. The gold saved can be used to create more siege weapons.
So overall, if playing as the Celts, Booming would be a good choice, as with their economic bonuses they can make up a rather big amount of resources, which will help them in the late game. It's also important to focus on Infantry (especially the Celts' unique unit, the Woad Raider), and Siege Weapons, as a combination of the Celts' two strongest type of units will make up a quite powerful army.