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"The natural formation of the country is the soldier's best ally... " -Sun Tzu, "The Art of War."
Chapter 2- Use the Terrain
Terrain shapes everything you do in a battle. It determines where you choose to build your base, set up your base defenses, hold ground which you have taken, and where to lure an opposing army to fight. Knowing the map is important; knowing how to use it to its full potential is vital to victory.
Open terrain: this is flat ground with little or no cover. Troops can maneuver without obstruction over and through this terrain, which makes speed and agility a major asset. The advantage is with an aggressive, mobile attacker on this ground: being able to outmaneuver an opponent here will ultimately lead to victory.
Conversely, this is extremely poor terrain to defend. The lack of obstacles to a dedicated aggressor makes it a poor choice to take and hold unless the defender is able to artificially create them (such as walls and gates).
Narrow terrain: this includes such places as valleys and bridges, where a broad front is impossible to maintain. Single or double file is usually the best an army is able to maintain. Therefore, it is a very poor choice for attack or maneuver by a superior force, and should be avoided by a large army if at all possible. Otherwise, clearing the way with massive bombardments is advised before traversing this ground.
A clever defender can hold such a passage with minimum effort and troops for a long time. This is perfect terrain to interpose between a superior force and a weaker force, as the full weight of the opposition cannot be brought to bear on you. Setting up defensive structures at your end of the narrow terrain can create a lethal bottleneck to an attacker that can utterly crush him.
Broken terrain: this is uneven terrain broken by several obstacles which can divide an army, provide cover, and turn battle into a chaotic affair with no clear advantage. Maneuverability is key here: being able to move around the battlefield freely can turn chaos to your advantage, as nothing can be more demoralizing than having a large force strike from the rear unexpectedly.
A slow-moving force will find itself hopelessly outmatched here; moving them around this type of ground is advised.
Elevated terrain: this is ground raised above the level surrounding terrain. It makes for an ideal vantage point and artillery position, and should be occupied and held before your opponent.
The configuration of the elevated ground could make it difficult to hold: if it is accessible from all sides, it may be next to impossible to defend. Nevertheless, if it isn't held, an opponent may occupy it, making it impossible to bypass in either direction.
Impassable terrain: this is terrain that cannot be moved along by ground forces. No battle can take place on this ground, so using it to protect one side of a base or other staging area is counselled.
In more modern settings, air units can bypass this ground and mount raids on you. Otherwise, impassable ground is the best defense you can get.
"...but a power of estimating the adversary, of controlling the forces of victory, and of shrewdly calculating difficulties,
dangers and distances, constitutes the test of a great general."-Sun Tzu, "The Art of War"
Reconnaissance: scouting the map is a key ingredient in planning how you will use the terrain. Once you have a scout in play, use it to quickly look over the area. Keep an eye out for places for defenses, and make a preliminary plan as to how you will move your army to the enemy base. Be aware that your opponent will also be doing the same thing, and make plans to thwart his advance.
Keep your scouting active. Know when and where he is moving, and move to meet him where you want to fight. When you are ready for attack, make sure you know where he wants to meet you for battle, and avoid it if at all possible.
Air units: many games have air combat, especially those in modern to futuristic settings (although some fantasy RTSs have air combatants). In Sun Tzu's time, any attack from the air was unimaginable, so this was never mentioned. For our purposes, however, the air can be treated as open terrain: speed and agility are key to controlling the battle for the sky.
Another consideration for your ground units is protecting themselves from the air attacks. For them, the sky is impassable terrain.
Naval units: this can be treated the same as ground combat. All the same conditions apply, except for impassable terrain: a ground army may be able to occupy terrain impassable by your navy. Keep this in mind as you maneuver them.
Conclusion: Knowing where you will fight is important. It will shape the tactics used and the units involved in the fight. Studying the terrain ahead of time will bring you a huge step closer to victory over your opponent.
(Next: Chapter 3: Base Building)