The Fort is a military building in American Conquest and American Conquest: Fight Back. It usually is the first building you can build as a European/American faction. It is needed to produce and upgrade 17th century infantry units, like Pikemen, Musketeers, Arquebusiers and the 17th century officer, drummer and standard bearer.
Like the Fortress and the Blockhouse II it will shoot with cannons on enemies once there are units or peasants inside. This can be risky at times, since it can also hit and set ablaze your own or allied buildings if they are in the way.