The Terrains from AW2.
On the battlefields of Advance Wars there are many different kinds of terrain. It plays an important part in a strategy, as each different kind has an effect, e.g. giving a defence bonus or impairing mobility. Though only ground and sea units are affected. An Air Unit will not gain a defensive bonus nor have it's movement restricted or be hidden in Fog of War. See Category:Terrain for a full list of all terrain types.
Defence
Each piece of terrain has a defensive rating ranging from 0 to 5 stars. The higher the rating the more defence it provides. This bonus must always be considered when placing units. It reduces the damage an attacking unit receives from a counterattack and increases the chance of survival of your units during your opponents turn. Placing units in prime locations could mean the difference between having a few damaged units to hold the line and having none at all.
Movement
The range units can travel may also be affected, except for air units, they always move unrestricted. A number is used to show how a type of unit moves over type of terrain. 1 means the unit can move unrestricted over that tile, such as roads, all buildings, shoal and open sea, a 2 signifies that a particular type can only move up to half its original range, etc. Mountain and river tiles are the most restrictive land terrain, only infantry and mechs may cross them. Forests and plains cause trouble for motorized units, more so for tires than threaded units.
Before Advance Wars: Days of Ruin, sea units only had reefs to consider.
Weather also plays a role in how far a unit may move, though it's effects have changed throughout the series.