Shields represent common raw materials in the countryside and the labor required to make useful materials from them. In essence they are a measure of production. The shields a city produces are used to complete its current project, which may be to build a military unit, city improvement, or wonder. When the production box is full of shields, the project is finished, the box is emptied, and a new project must be started. Some cities lose shields to waste. Shield production depends upon the terrain within the city's radius, because citizens laboring there produce shields. When the City Display is open, you can see what the citizens are producing. A citizen working Produces this many shields Grasslands 0 1 if mined. Outcropping 1 2 if mined. Plains 1 2 if mined. Hills 1 3 if mined. Mountains 1 3 if mined. Tundra 0 1 if mined. Desert 1 2 if mined. Forest 2 cannot be mined. Marsh 0 cannot be mined. Volcano 0 cannot be mined. Note that some natural resources provide bonus shields as well.