08 September 2011

Fundamentals of Game Design, ch 14: Strategy Games

Strategy = planning series of actions against the opponent(s). There's usually a large variety of potential actions. Analysis paralysis happens when players have to wait for the other to be done thinking about his next move. In RTS, decisions have to be taken quickly. The basic mechanics of strategy games involve units and the factories that produce them. There can be diplomacy if the scenario is long enough. The attributes of units can be: (max) health, (max) armor, atk, ammo, aspd, area of effect, range, and others ...

Melee unit's value (eg footman) = max HP * shot power * aspd * accuracy * range * walk speed
Ranged static unit's value (eg tower) = max HP * shot power * aspd * accuracy * range^2/2. Squared because the area defended increases quadratically. Halved because units can generally only cover one side/half, the one side towards the enemy.

Most RTS make little use of line of sight and terrain realism; a unit on top of a hill could see further, a unit behind a hill could be hidden from those behind it, units have atk/def/... bonus when they are in their favorite terrain.

Gold is too important for war for mines to be spread out randomly. Spreading randomly means randomly-chosen players get a random advantage from the start, which is unfair. Mines randomly spread also means that players will hunt for mines instead of fighting strategically against each other. Large mines should be positioned symmetrically if they are decisive. Random mines should be smaller and not be decisive for winning.

Lanchester's laws: 1- In melee combat, army strength is proportional to number of units because each fighter can engage only one enemy at once. 2- In ranged combat, units can concentrate their power on a single enemy. When 1 unit is attacked by 3 archers, its life decreases 3 times faster, and it can only attack one of them, ie 1/3 of the enemies. Therefore, 3 ranged units are 3*3=9 times stronger than a single unit (whether ranged or not).

Imagine the tight and strong negative feedback loop: "wounded units attack weaker". Then players have to play sub-efficiently all the time, and they're going to wage attrition skirmishes, not big epic battles.

Do not block or forbid certain units or skills in a particular level. Instead, make the level so that this unit or skill is very inappropriate. Ex: to prevent the player to use air units, give the opponent lots of anti-air missiles. To prevent the use of iron, set the victory condition to "collect lots of iron".


Upgrades can apply to a single unit (veteran units have +1 dmg), all units of a particular kind (grunts get +2 atk), all units of all kinds (Iron Age = +2 def to everyone), or even the whole mechanics (production speed divided by 2). Upgrades can be unlocked by spending resources, time, achievement, ... The tech tree can be:

  • totally open: player spends point in any tree she wants, like in WoW
  • constrained: finish one branch to start another
  • fixed: once player starts in a tree, only this tree is available for the rest of the game

Production and distribution

Production process should be concrete and visible, while the distribution process should be abstracted. Ex: plant farms for food = concrete food production. No supply lines as long as there are roads = abstract supply distribution. There are different shades and techniques for supply lines: troop caravans between cities in Heroes of Might and Magic IV, trade caravans between cities in Civilization series or Age of Empires, or Armored Personnel Carrier that transports troops and fuel in Advance Wars. Another approach is to have cities within a zone of influence to be supplied: cf the influence zone mechanics of Populous (you can only build in your area of influence), Civilization, and Black and White (you can only exert your god powers within your zone of influence).

AI techniques

  • Game tree search, as used in games where there is a finite number of possible moves, such as Chess or Checkers. Problem: the search space increases exponentially, which is a problem for games like Go which have too many possible moves.
  • Neural networks: patterns are identified after repeated exposure. Problem: training takes time, the system can hardly be tweaked, and adding new patterns means re-training the whole system.
  • FSM: units transition from one state (walking) to another (shooting) because of events (enemy appeared in sight). Problem: the unit can only be in one state at once.
  • Hierarchical FSM: FSM of the commander gives orders such as "take this hill" to captains. Captains have an FSM that gives orders to soldiers in the squad, such as "cover fire" or "make diversion". Each soldier has its own FSM as well: "if I see an enemy, run for cover". hFSM can create emergent behavior.

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