Terrain

§ In Civilization IV, the world is made up of land and water "squares." Each square is designated as a specific "terrain type": for example, desert, hills, plains, coast, ocean, etc. Each may contain additional "features": forest, jungle, oasis, and so forth. A river may run along the edge of the square as well. All of these elements help determine the usefulness of the square as a place to build a city, as well as how easy or difficult it is to move through the square. And, of course, terrain may have important effects on any combat taking place in a square. The charts below display all of the effects of terrain on the game.
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Land and Sea

§ All squares are either land or water squares. Water units cannot enter land spaces (except for coastal cities), and land units cannot enter water squares (unless carried aboard Transport Vessels). Some air units can enter both types. (See Air Units for details on air units.)
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Terrain Effects on Movement

§ Some terrain is more difficult to move through than others. See Movement for details.
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Combat Effects of Terrain

§ When two units fight each other in open terrain, both fight at their standard Combat Strength. Some terrain gives benefits to the defender - for example, most units get "defensive bonuses" when they are occupying hills or forests. Such bonuses serve to temporarily increase the defender's strength for that battle. For example, a spearman at full strength (with a strength of 4) who is attacked while in a forest, gets a 25% defensive bonus; for that battle, his strength is increased to 5. However, if that spearman were to attack, he would get no bonus from the forest - defensive bonuses only go to the defender. See the Combat entry for more information.
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Terrain Productivity

§ Cities draw sustenance from the squares they occupy, and from nearby surrounding squares. Each terrain-type and feature provides a certain amount of Food, Commerce, and Production to cities; the higher these values, the better for the city. Note that Resources will further increase a square's value, as will various worker Improvements. See Cities for more details.
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Rivers

§ Unlike other terrain features, rivers run alongside land squares, not in them. Rivers can affect all squares they run adjacent to. Rivers negate the movement benefit of roads until you acquire the Construction technology. Rivers also reduce an attacker's combat strength when the target is on the other side of the river. Rivers improve the commerce value of adjacent squares as well.