The Stable in Age of Empires III trains and upgrades a variety of cavalry units, depending on the civilization. Cavalry is primarily used to counter different unit types such as artillery and infantry.
"A stable is a building in which animals (typically horses) are housed. Horses were vital to survival in the New World and every inn, home, and town had to have stables to shelter these important animals. Horses were used for travel, plowing fields, and for warfare up until the mid-twentieth century.
The Quartering Acts, passed by British Parliament in 1765, allowed British soldiers to occupy living space in just about any building they deemed suitable for housing."