Rocket

Rockets or the Congreve Rocket is a British and Chinese Artillery piece that is fairly expensive and is featured in Age of Empires III. They can be produced for free in a Factory, the Chinese gain access to them only from constructing a Wonder which also produces them in a similar fashion (free).

Hidden Cost
Training these units at a factory requires 98 seconds, 93 with Team Engineering School sent by a Chinese ally. The average gather rate of a Factory is 5.5 every second (before the building upgrade (+20%) and Home City Card, Market, etc upgrades). This means that Heavy Artillery from a Factory, while appearing to be free, will have a cost dependent upon how effective the factory gathers a needed resource. In many cases this cost can exceed the base value (600 resources) of the unit.