Each city can be producing one thing at a time, either a building, a Wonder of the World, a spaceship element, or a unit. Each turn the city brings in production points from plains, mines, forests, and other map resources, possibly multiplied by buildings in the city and Wonders elsewhere in your empire. Some of those production points may be automatically expended by units for which the city is home; see Government Types. Any production points remaining are devoted to building the city's current project.
Later sections of this manual will describe the buildings, units, spaceship elements, and Wonders which a city may produce. Your ability to produce each depends on your technology level and the location of your city (e.g., only coastal cities may produce sea units). Each thing you may produce has a cost, given in production points, which must be paid to complete it. The city window will have a box with as many slots as there are production points required; production points already spent on the project are marked so that you can see the progress.
When a city is created, it is automatically set to produce the best defensive military unit available to your civilization. You may change any city's production at any time, regardless of the state of progress of the current project. However you will lose half of what was already invested in a project if you switch from a unit to a building or a Wonder, or from a building or a Wonder to a unit.
It is possible to spend gold from taxes to speed the completion of something being produced. To "Buy" something causes the required number of production points to be filled in completely, at the expense of a certain number of gold coins from the national treasury; the building, unit, or Wonder is then completed at the next update. The cost to buy early completion of a unit is substantially more gold coins than the production points remaining, and doubled if no production has yet been invested on the unit; a city in disorder (see Population) cannot buy a unit. The cost for a building or Wonder is twice the production points remaining, doubled if no work has yet been done, doubled again for a Wonder. Once you have bought the completion of something, you may not change the city's production for that turn.
When a unit has been completed, it simply becomes visible on the map and available for commands, and the city automatically begins building another unit of the same type. If the new unit is a Settler or Engineer unit, the city also loses one population point. When the construction of a building or Wonder is complete, the game picks a different building, unit, or Wonder for the city to start working on; you must pay attention to the message announcing the building completion and decide if you like what the game chose for you.
If the amount of production it took to do the last bit of work on a project left some work left over, it is not lost. The manhours and resources left to spend are immediately put into the new project.
If a city is producing a unit which becomes obsolete before it is finished (e.g., a city is producing a Legion when you discover Gunpowder), the game automatically changes the city's production order to the corresponding appropriate unit (in this case, Musketeers).
Cities can also upgrade obsolete units, either individual units or an entire unit type. The unit must be in the city, and must be of a type that is obsolete (e.g., Legions in an era of Musketeers). For an immediate cost in gold, the unit is upgraded into the new type.