The Celts are an offensive civilization that focuses primarily on infantry.
Their infantry move faster than those of other civilizations and their unique unit, the Woad Raider, is the fastest infantry unit in the game. They also have very effective siege weapons. They have access to all siege weapons except for Bombard Cannons and they have a faster attack speed. In addition their unique technology, Furor Celtica, greatly increases the hitpoints of their siege weapons and makes them quite difficult to destroy. Economically they have a small advantage. They can gather wood faster than other civilizations which frees up some population space since fewer villagers have to be tasked to gather it.
Defensively they are average, lacking many defensive upgrades and Bombard Towers. Their cavalry is decent but is nothing special (they lack Bloodlines). They are able to upgrade to Hussar and Paladin but they lack Camels. However their lack of camels is compensated by the faster speed of their spearmen. They also have poor Monks missing many technologies at the monastery.