The Celts are a Western European civilization in Age of Empires II based on medieval Ireland and Scotland. They focus on infantry and siege weapons.
Their descendants are the Scottish and Irish nations. Wales is also considered a Celtic nation, but is not represented by the Celtic civilization. The Bretons are likewise a Celtic tribe in Europe, but their culture and Duchy was assimilated by France during the late Medieval Age, and is more represented by the Franks.
Wales and Ireland came under control of the English nation (another partial descendant of the Britons) during the 1200s and 1500s respectively, with the Scottish parliament voting to merge with the other three in 1706 during a time when a Scottish king inherited the English throne, which created the United Kingdom. The United Kingdom is featured in Age of Empires III as the British civilization. The Celts do not reappear as such, but Morgan Black, the Highlander, and the Irish Brigadier are descended from them.