That said, the most important change to the terrain system involves goods. In previous Civ games, map goods functioned as "bonus" tiles, providing extra food, shields, and trade to the city working that tile. This feature is still present in Civ III; for example, a tile with wheat provides two extra food. However, certain goods, known as luxuries and resources, provide extra benefits to your cities. Luxuries, such as spice, silk, and diamonds, increase the happiness of your people. Resources, like iron, oil, and uranium, allow the construction of specific units and buildings. Therefore, resources become a source of strategic advantage and should be protected as such.
Moreover, these resources and luxuries can be shared between cities via your trade network. For example, if there is an iron tile anywhere within your borders, all of your cities that are connected to that tile via road will have access to iron. Furthermore, resources and luxuries outside of your borders can be made available by building a colony. Trade networks can also cross air and water via airports and harbors. Finally, resources and luxuries can be traded with other civs if the goods are connected to your capital via road, airport, or harbor. If you consider that the culture of your civilization directly impacts the reach of your borders (and therefore your ability to harvest resources and luxuries), you begin to see how interconnected the culture, trade, diplomacy, and combat systems are in Civilization III.