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.
Only land and naval units are affected, an air unit will not gain a defensive bonus, cannot hide in terrain such as Woods , nor have it's movement restricted.
See Category:Terrain for a full list of all terrain types.
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 should always be considered when placing units. It reduces the damage taken from a counterattack by an attacking unit and for units getting attacked themselves. Placing units in prime locations could mean the difference between having a few damaged units and having none at all.
The range units can travel may also be affected. A number is used to show how a type of unit moves over a type of terrain. 1 means the unit can move unrestricted over that tile, such as Road , all Building , Shoal and open Sea. 2 signifies that a particular unit 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. Woods and Plains cause trouble for motorized units, more so for wheeled units than threaded ones.
Fog of War
During Fog of War some types of terrain gain additional effects.
Placing Infantry or Mech units on mountains will cause their range of vision to increase from 2 tiles to 5 tiles, allowing you to keep tabs on your opponents while your unit is relatively safe.
Vision will also not directly apply to woods, reefs or sea fog. They will always remain covered with FOW unless there is a friendly unit next to it. This effect provides several advantages: Placing units in such tiles effectively makes them invisible to the enemy until they place a unit next to it, allowing you to move units unseen, attack from right under their nose or retreat to a safe location where they will need atleast two units to attack yours.
There is also the danger of ambush. An ambush occurs when a unit moves across a tile which is already occupied by a previously unseen enemy unit. The moving unit automatically stops and cannot take orders for the rest of that turn. Moving through terrain that negates vision, such as woods, together with the lowered movement, makes for a risky manoeuver when unaware of enemy positions.
It should be noted that a few aesthetic features may give away some information. Battle animations when turned off only show the damage being done to the targeted unit, but when turned on the units involved on both sides are displayed. Indirect units and hidden units such as the submarine, as well as the terrain they occupy may be revealed this way whereas the opponent otherwise may not have known where the attack came from. Similary, moving units make different sounds depending on what kind of unit they are. They can also pass through tiles of terrain where the opponent does have vision, showing a brief glimps of the unit.
There are ways of dealing with FOW:
The recon unit combines a large field of vision with a great movement range making it invaluable during FOW encounters. Its machine gun can take out Infantry and Mechs so they cannot be placed on Mountains, thus denying your opponent vision.
The Days of Ruin Flare unit allows you to shoot a limited amount flares a certain distance, making an area of two tiles around the target completely visible, including woods, reefs and sea fog.
Weather conditions may change during battle, causing different kinds of effects for the duration of the weather. Some COs can actively change the weather using CO Powers, including on maps where weather change was initially turned off.
Snow reduces the movement range for units, making some maneuvers increasingly difficult, such as wheeled units over Woods. In Advance Wars: Dual Strike snow doubles Fuel usage instead, which poses a threat to air and naval units.
Rain has also had different effects. In Advance Wars it inhibits movement to a lesser degree than Snow does. In later installments it reduces vision when FOW is present and for the duration of the rain causes FOW to appear if it was not present.