The Capitol is a building in Age of Empires III that becomes available once the Imperial Age is reached. It is the source of a number of very powerful improvements meant to make the side that can afford them so powerful that they can end an otherwise stalemated game. Each improvement featured in this building provides a distinct purpose in each area of the game. Some of the most important technologies available in the game are exclusive only in this building so it is advised to utilize it whenever you can. Some of these technologies are meant to improve the economic condition of a civilization.
"A capitol is a building or set of buildings that houses government offices. usually the legislative branch. Four buildings have served as capitol buildings in the United States. The first was the Maryland State House in Annapolis, Maryland. That building still stands and is currently used to house the Maryland General Assembly."