Resource Guide
In Cities, resources such as Food and Knowledge are combined to form low level assets such as Concepts and Skilled Labor. These low level assets are then combined to build high level assets, such as Buildings and Machines. It requires 5 high level assets to build a city: one of each type and a fifth to define the type of city. The resource and asset combination guide is here:
Graphic depiction of combining resources to form some low level assets.