Enemies. Monsters. Sprites. AIs. NPCs. Creatures. Villains. What are they? More specifically, in computer games what are they? Hurdles that must be overcome in order to reach the exit to the level? Simulated opponents that try to kill the player or stop his progress? Beautifully rendered characters with interesting attack, fidget and death animations? Collections of stats?

All of the above, I suppose, but one of my pet peeves is that many game designers seem to have forgotten (or never learned) how to make interesting and distinctly different enemies. If a game has a line-up of creatures that look awesome but simply represent an ever-ascending set of numbers in several character attribute fields, then chances are that game will get boring quickly. At the very least, it will not be as exciting as it could have been, had the designer planned out the creature's role in the game at an abstract level.

As with weapon differentiation, if each enemy has a particular function in the game world (and that function is obvious and/or something the player can learn about over time through observation), then the player can decide how to deal with each enemy. That enemy characteristic then becomes part of the game’s dynamics. Alternatively, if all enemies do basic variations of the same thing (such as race forward and inflict damage on the player), the player is apt to grow bored. The key, therefore, is to design game units that will make the game more interesting.

Case Study of Enemy Units: Defender

Let’s look at Williams Electronics classic arcade game, Defender, and examine three of the units in this game: the lander, the mutant and the bomber. By examining the ways in which these three units differ within the game's framework of actions and mechanics, you can see some of the ingredients necessary for intriguing enemy behavior.

• The lander flew slowly and occasionally fired a shot at the player. Given the chance, the lander would descend to the surface below to pick up one of the humanoids that the player was supposed to be protecting. When a lander grabbed a humanoid, it would rise toward the upper levels of the atmosphere, suddenly firing like mad; when it reached the top of the screen, the humanoid was destroyed and the lander would become a mutant.
• The mutant was faster than the lander and much more aggressive – it always made a headlong attack when it spotted the player. The mutant also had an erratic flight pattern, making it’s movements less predictable and harder to evade.
• The bomber was slow and peaceful for the most part, but it left a trail of bombs that hung in space; if the player contacted one of the bombs, his ship was destroyed.

Ahh, Defender.

Discounting their appearances and fictional identities completely, think about how different these three units are. The behavioral attributes of three units that concern the player essentially are "movement rate", "flight type", "aggression level" and "special ability". The "special abilities" (an arbitrary term) for these units were consume humanoids then change unit types (the lander), pursue the player aggressively (the mutant) and leave a trail of explosives (the bomber).

(Note: The lander's ability to consume resources – the humanoids – is brilliant. The gameplay is made dynamic because the lander changes from a relatively weak enemy to really critical enemy as soon as it picks up a humanoid. The player must immediately switch gears and deal with the threat of the lander becoming a mutant, or suffer the consequences.)

In many games (old and new), the enemy design is nowhere near as interesting or distinct as in Defender. For example, in a certain game Creature One can move at a game speed of 50, does 37 points of damage in an attack and has 100 hit points, while Creature Two can move at a game speed of 75, does 12 points of damage in an attack and has 30 hit points. In some games, this is the extent of the enemy differentiation. The problem is that this sort of design does not cause the player to dynamically adjust his play. It does not force the player to make critical decisions about how to react to a game enemy; the reaction is generally always the same, since the enemies are fundamentally the same.