A tank factory is an original building from the game, The Conquerors. It can be seen in some of The Conquerors 3 matches.
A tank factory is used for making all in-game tanks at the moment, except for artillery. People like to make anti-air tanks or heavy tanks since they do accurate damage to an army later on in the game.
Due to tank factories only having the ability to make tanks, it is a good idea to keep it near a wall/gate. Or surrounding an priority building making it harder to kill the main building. (this technique is called "Housing")
Explosive Tank: $60
Light Tank: $70
Heavy Tank: $90
Anti-Air Tank: $90