Enemy Design

Enemy design is the choice of what opposes the player and how it behaves. The designers quoted here made those choices at the arcade cabinet, the Famicom and the Xbox: Tomohiro Nishikado gave the targets guns, Toru Iwatani gave the ghosts personalities, Eugene Jarvis gave the robots randomness, Shigeru Miyamoto gave the Koopalings faces, and Bungie let the Grunts panic.

Fast facts

Enemies that shoot back: Space Invaders (1978) — earlier shooting games had “passive targets”, Nishikado said.
Enemies with personality: Pac-Man (1980) — “All of the ghosts have different AI algorithms that reflect their comical personalities” (Iwatani).
Tuned by play: Robotron: 2084 (1982) — the Grunt took a week; “several weeks were spent tweaking each enemy with extensive play testing” (Jarvis).
Scripted or emergent: Halo (2001) — behaviours that “combine in a more emergent fashion” rather than scripted encounters (Griesemer).

Enemies that shoot back

Nishikado’s account of Space Invaders, given to Retro Gamer in 2013, puts the enemy at the centre of the design: “most to that point had the player shooting down passive targets to rack up points. Space Invaders had the enemies shoot back; I think that was especially exciting to the youthful arcade audience.” Andy Walker, who later founded Taskset, remembered what that felt like from the other side of the cabinet: “an automatic crescendo as they marched down the screen”, and the invader who turns the upside-down Y in PLAY the right way up, “the first ‘attitude’ and ‘soul’ expressed by a videogame character that I can remember.”

Enemies with personality

Iwatani told Edge in 1998 that Pac-Man began with eating, and that “After that, I decided to add the element of four ghosts as enemies. Just moving around an open field would be too much freedom and no fun, so I set up a maze environment.” He refused to treat Pac-Man as the only character:

I feel that it’s important not to just think of Pac-Man as the only main character in the game — the ghosts play an equally important role. All of the ghosts have different AI algorithms that reflect their comical personalities. It’s sort of like asking whether Tom or Jerry is the star of the ‘Tom and Jerry’ cartoons.

The ghosts are “cute (or playful)” rather than frightening, and the power pellet turns the pursued into the pursuer, a reversal Iwatani said “makes the game much more dynamic”. How the four algorithms differ is the subject of ghost AI.

Miyamoto’s approach in Super Mario Bros. 3 was the same instinct at Nintendo. Nintendo Power’s 1990 making-of reported that “The most important new enemies created for SMB 3 were Bowser’s children. Each was given a unique look and personality.” His favourite was the Chain Chomp: “This is a strange enemy because it is chained and can’t get at Mario, which probably accounts for its vicious behavior,” which the magazine traced to a “bad experience” with a dog in his childhood. The old cast came back altered — winged Goombas, giant Koopas, new Piranha Plants, more Hammer Brothers — “to add spice to old familiar [va]riety.”

Tuned by play

Jarvis’s Robotron enemies were written fast and tuned slowly. “We had the Grunt finished in the first week. Its AI was to always seek the player on the shortest possible route and ignore anything else in its path. In all though, several weeks were spent tweaking each enemy with extensive play testing.” Nothing in the game is deterministic:

There’s always a random factor added to any behaviour for more interest. It’s amazing how players will associate random events with mood or intelligence of enemies. We constantly want to make sense out of nonsense. We see the game as being pissed off or angry when there has just been a bad lot of random numbers drawn.

The testers were the designers. Jarvis thought that was lucky: “we were good but not great players, so we avoided the tendency by expert gamers to make things impossible for the average player”.

Scripted or emergent

Twenty years on, Bungie’s Jaime Griesemer described the same choice for Halo in terms of scripting. The team could “rely on heavy scripting and planning out every single encounter”, or build “lots of different behaviours” that “combine in a more emergent fashion”; scripted games work, “but you always fee[l] a litt[l]e like you are watching a movie”. Halo’s AI programmer, Chris Butcher, spent a period doing nothing but adding behaviours, so that a familiar encounter “would do something completely different” — a Grunt throwing a grenade, or a room of Grunts running for the doors when their leader dies. Replay variety came less from random placement than from the AI answering the player’s tactics: “if you change your tactics, the AI will change in order to compensate.”

Playing by its own rules

The critic’s test for an enemy is consistency. Steven Poole’s Edge column of October 2001 named the failure “incoherence — that [m]oment when you realise the game isn’t actually playing by its own rules”, and gave Red Faction’s guards, who ignore a disguised player until a drawn gun gives them “a sixth sense for weapons”, as the example: “a rule that you work around sulkily, rather than one whose challenging strictures you actually enjoy.” He also stated the arithmetic every enemy designer works under: a realistic enemy would make the game “completely unplayable”, so “it is arithmetically necessary for the player to be given deadlier abilities than the enemies”.

Making meaningful opposition