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Living Worlds: The Ecology of Game Design

In this fascinating article, former EverQuest designer Carter reveals "...three simple guidelines you can use to make your game worlds that much more believable, and therefore that much more exciting to play through", citing games from Half-Life to Out Of This World.

Kevin Carter, Blogger

May 8, 2007

22 Min Read

  I have a great time playing through all sorts of game worlds—blasting randomly placed enemies, collecting scattered power-ups, earning points, gaining levels, all en route to some nefarious “main boss.” Usually the game world is very transparent though, meaning I can tell I’m in a game from the moment I pick up a controller to the moment I put it down again. As a player, I’m not exploring environments, I’m beating levels. I’m not fighting aliens, I’m defeating scripting.

This isn’t always the case. Sometimes I can forget I’m playing a game. Sometimes I feel I really am crawling through an air vent in the Black Mesa Facility. Sometimes I think I’m actually looking for the orc chieftain who has been leading the attacks on my village. Sometimes I—

Wait! Whoa, let me back up for a second. I’m afraid I’m giving you the wrong idea here. I’m not one of those immersion freaks who speak in character in MMO general chat. (“Lo, I am Beragond, son of Herengar, from the House of—.”)

Please don’t think that.

All I’m trying to say is that sometimes a game feels like levels and sometimes it fells like a world. Do you know what I mean? The difference is subtle but important. I have fun playing either way—the game play “works” regardless—but the times that I feel I’m in an actual world, as opposed to digitized geometry, are the times I remember most. Those are the experiences that keep me gaming.

I’ve compiled notes on the conditions that enhance, or at least encourage, the feeling that a game’s environment is a real place, that it may theoretically exist somewhere out there and is not just a collection of levels built solely for my amusement. Surprisingly, this kind of immersion has little to do with graphics (though good graphics never hurt, they are not the focus of this article). It has more to do with subtle elements borrowed from the real world. My notes on well-constructed believable game worlds, derived from many, many hours of video games played since I was a tricycle-riding youth, are summarized below into three main points. Strangely enough, all three points relate directly to ecology. Think of this article as an attempt to preach the basic ecology of video game world building.

If you design sports games, or maybe puzzle games, you can probably give this article a skip. But if you’re a world builder, the kind of designer who strives to create exciting and believable environments, no matter how alien or bizarre, then read on. This article will give you three simple guidelines you can use to make your game worlds that much more believable, and therefore that much more exciting to play through.

1) Creatures are Part of Their Environments

Begin the level design process with your Non Player Characters (NPCs) in mind

I’m going to use the term “ecology” very loosely here, so loosely that it need not necessarily apply to creatures that are alive in the strictest biological sense. In other words, and this disclaimer is really only necessary in a discussion of video game world building theory, the ecological patterns I’m about to describe can apply to creatures that are not specifically alive, creatures such as undead zombies, self-replicating robots, puddles of sentient ooze… or whatever other imaginative aberrations your particular game may place between the hero and the much-feared main boss!

In any case, regardless of your particular brand of bad guy, you should always remember two things. 1) creatures are part of their environments and, as an ancillary rule, 2) creatures shape, and are shaped by, their environments. What I mean here is that creatures do not exist in a vacuum. When a player encounters a creature in a game world, that creature should, at a glance, appear to belong in its environment. Communicating, or at least implying, the connection between a creature and its environment is often far easier then most game designers realize. There are several ways to do this including: environmental associations, contextual events, or even simply the visual appearance of the NPC model itself.

The easiest of the tools that can be used to tie creatures into their world come from environmental associations between the creature and the surrounding game geometry. In other words, wherever a creature appears in game, it should be nearby an object (or objects) typically associated with that creature type.

In the World of Warcraft, for example, the Silithids (a race of giant-insect creatures) build hive mounds and strange claw-like chitin towers wherever they go. These architectural structures even look like silithid-shells, implying that they were made, or perhaps grown, by the creatures that live in and around them. The end result is that whenever players see these structures from afar, they know silithids are nearby—this kind of logic makes sense to explorers of the World of Warcraft—and so they are not surprised when they encounter an angry hive of giant bugs near the chitin towers.

A Silithid hive in World of Warcraft

The silithids are just one example of the many races in WoW that have unique types of game geometry which are used to visually tie creatures to their environments. World of Warcraft also utilizes unique dwellings for the furblogs (hollow logs and tribal banners drawn on animal skins), as well as for the murlocs (stilt houses built along shorelines). In fact, just about every sentient race in WoW has a unique type of architecture. Even the oozes have their green mud-puddles to crawl out of.

The end result is that very few creatures in WoW appear out of place. Nothing just appeared there out of nowhere. In fact, environmental associates give the appearance that these creatures have inhabited their environment long before the player arrived, thus giving the player the idea, at least subconsciously, that they are moving through a living, breathing world with its own patterns and consistency. More importantly however, the player is rarely surprised by the type of monster they find within any particular environment, which in turn reinforces the believability of the world as a whole. Compare this to the alternative—which is filling a forest with a randomly-placed medley of goblins and griffons or, worse yet, groups of goblins and griffons standing in an open field—and the advantages of environmental associations really stand out in their ability to convey a believable ecosystem. I’m not saying your game can’t have roving packs of creatures in otherwise unspoiled wilderness, I’m just saying that these kinds of encounters should not be the norm.

  Another way to communicate the connection between a creature and its environment is via contextual events. “Contextual event” is a fancy way of referring to any action performed by a NPC within its environment, usually, but not necessarily, an action triggered by scripting. This is really simpler then it sounds.

Remember all the crazy encounters in the original Half Life? Remember how head crabs and biogun-wielding alien invaders and black-op marines were popping out of god damn everywhere? Well, despite the wide range of bad guys to fight in Half Life, and despite the fact that none of these bad guys were native to the Black Mesa Facility (the setting of Half Life), very few of them seemed particularly out place when you, the player, encountered them. This is because the designers of Half Life used contextual events very effectively.

Allow me to elaborate.

Essentially, and I’m hope I’m not spoiling the game for anyone, the bad guys in Half Life were invaders from one of two places: either a government military base or the alien dimension of Xin. Throughout the game, the designers set up contextual events to remind you, the player, that the base is being invaded by two outside forces: the marines and the aliens. You see marines rappelling down from circling helicopters to join the fray. You see them setting up forward positions, unloading trucks full of supplies. You see them setting up scout patrols and you even hear them discussing their mission on their radios as you crawl through the (gratuitous) ventilation shafts of the Black Mesa facility.

Likewise on the aliens’ side, you see them warping into the base through strange green rifts, the same strange green rifts you accidentally tore open during a failed experiment in the beginning of the game. By witnessing these events, you as a player have a clearer understanding of what each enemy is doing in its particular location of the Black Mesa facility. Thus, though they are non-native to the game’s setting, these NPCs do not seem out of place when you encounter them, guns drawn and teeth bared, ready to brawl.

Contextual events also serve another important function when it comes to brining a game world to life: they enhance the relationship between an NPC and its environment by implying that, like the player, the NPC can also manipulate and interact with the world. It’s one thing to encounter a group of marines hanging out in a warehouse, waiting with their guns drawn for some hapless player to walk by, and it’s another thing entirely to have that same group of marines arrive via a freight elevator as the same hapless player is exploring that same warehouse.

Contextual events make a game world and the creatures in it feel far more dynamic then they would otherwise. And, as we all know, dynamic worlds are generally more believable. Even if the elevator mentioned in the example above is scripted to arrive exactly twelve seconds after the player enters the warehouse, it still makes the game world seem more unpredictable and realistic. Indeed, a player may even be more wary of closed elevator doors later in the game, fearing NPCs may come pouring out at any moment, which is exactly what you, as a game designer, want.

Interesting side note: the designers of Half Life also effectively used architectural associations to tie their creatures to the game’s environment—for example, you can tell marines are nearby when you encounter sandbag barricades, and you know aliens are nearby when you see green Xin plants growing along the walls and floors of the Black Mesa facility—but we’ve already discussed effective usage of architectural elements, so let’s move on, shall we?

A third way to imply a relationship between an NPC and its environment is via the visual appearance of the NPC model itself. There are many ways to do this, and I’m not going to indulge in a lot of explanation here because this concept is relatively straightforward, but the main idea is to make an NPC look like it belongs in its environment.

There are a number of ways to do this. Rather than elaborate on all of them, I’m just going to list a few examples to give you an overview. For starters, swamp troopers in Star Wars Galaxies shun their traditional white armor and instead wear green camouflaged suits to blend in with their surroundings. Treants (tree creatures) in EverQuest have three different mats—oak, willow and deadwood—to allow them to blend into a variety of zone geography. The flames around the skulls of the lost souls in Doom III have roughly the same appearance as the fires of hell, which you actually see in the later levels (again, I hope I didn’t ruin the game for anyone). The giant in Prince of Persia has skulls on his belt that are properly sized with the same skulls in the creature’s lair, implying that the creature has preyed on many past heroes like yourself. It’s little touches like these that cement the relationship between an NPC and its environment, and by “environment”, I mean specifically the level you as a game designer are trying to convey.

Doom III's fiery disembodied lost souls

Also, while we’re on the subject, it might be worth your time to peruse a zoology textbook to learn about what types of animals live in certain environments here on earth. In far too many games, I’ve seen creature types in environments where they generally do not belong. Bugs in the arctic are one persistent example that continues to puzzle me. So too are giant carnivores in the desert. I’m not going to dictate the biological laws of your game world to you; I’m just saying you might have better luck conveying a believable ecosystem if you stick to the crudest of principles laid out by 3.5 billion years of earth’s biological evolution. Just a thought...

  2) Creatures are Territorial

Congregate Creatures in Groups Around Resources

A desert environment is full of barren sand… until you get to an oasis at which point there’s life everywhere: palm trees, lizards, grass, ferns, the works. That’s because unlike the rest of the desert, the oasis has a valuable resource, specifically water.

I chose the oasis example because it’s the best way to visualize the next concept that should be used by video game world builders, which is this: generally, life tends to congregate around resources. Humans live near roads. Frogs live near flies. Whales live near plankton. Zombies are found near graveyards. It doesn’t really matter what animal we’re talking about. The point is that every creature a game designer can think up probably has a resource that it can use. Your job as a game designer is to place that creature near to that resource. I know it sounds simple, but it really goes a long way to conveying a believable game world.

As much I hate to be the fanboy who keeps bringing up World of Warcraft, I must say this: WoW is probably the most consistent game out there for demonstrating the world building power of resource-oriented game population. Most creature types throughout the game are arrayed around specific resources, and resources can mean anything, including, but not limited to, farmsteads, mines, lumberyards, water sources, giant glowing power crystals, trade routes, and so on, and this idea probably comes from the game’s predecessor, Warcraft III (a real time strategy game that involved, to a large degree, the gathering of various resources).

Regardless of the reasons for such deliberate placement of resources, the end result is that very few creatures in the world appear to be out of place. Humanoids stand near crates of supplies. Amphibious murlocks hunt near the shore. Heck, even the vultures flock around dragon corpses. Everything is territorial around resources!

Another game that demonstrates this principal very well is Battlefield 2 (and probably Battlefield 1 as well, though I’ve never played it). The types of vehicles and stationary defenses that spawn around any given military base in the game are all dependent upon which army controls the base (i.e. Chinese, American or Iraqi). Although this game mechanic is fairly standard for capture-the-flag-style first person shooters, it’s worth mentioning here because it really helps convey the fact that these bases are resources that need to be controlled. “Why is the Iraqi sniper taking pot shots at me? Oh yeah, because I control the ground mounted missile launcher that he wants for his team.”

Territory played a significant role in Interplay's alien adventure Out of This World

The game Out of This World (released in the early 1990s; don’t worry if you don’t remember it) is one of the first to star a dimensionally “shipwrecked” scientist who must struggle for survival. In the opening levels, the player realizes that he has arrived on the territorial fringes of a hostile alien empire. Throughout the rest of the game, the player moves in and out of various wilderness and urban areas in an attempt to survive. After running through a few levels, you have a clear sense of what kinds of creatures are lurking in each environment you pass through (sewers, caves, wilderness and cities) as well as an understanding of how each creature will attempt to eat/kill you.

Not surprisingly, these creatures tend to organize themselves in groups around resources. The humanoid aliens populate military bases which house huge streams of energy (that you can use to recharge your stolen laser pistol) while the debris eating pit-mouth aliens lie in areas where debris (i.e. you the player) are likely to fall. This is especially effective because the game uses encounters with alien creatures relatively sparingly. By the last level you, the player, have a clear idea of what creatures you will find based on what resources are nearby.

As the astute reader has by now realized, the principle of congregating creatures in groups around resources (the second point of this article) is very similar to the principle of making creatures appear to be part of their environment (the first point of this article). If fact, they are so similar that the author must concede that they are in fact the same idea, and the he merely changed the wording slightly and presented this idea twice to fully drive the point home.

Creatures are part of their environments because their environment sustains them.

Get it?

Got it?


  3) Creatures Organize Themselves into Innate Hierarchies

Arrange like creatures in a hierarchical manner

Chex Mix is a ubiquitous party snack that is also damn tasty. It contains—in varying concentrations—bagel chips, pretzels, corn chexs, wheat chexs, and some squiggly breadstick looking thing, all of which are mixed together in a large bowl. Chex Mix is good because you can grab a handful of it and munch away and basically get a different texture with every bite.

I mean no disrespect to the good folks at General Mills when I say this, but the Chex Mix model—which basically boils down to “everything mixed together is good”—makes for questionable level design. Variety is the spice of life, this is true, but when trying to place base population in a game world, unnecessary variety can be a bad thing. Too many different creatures in the same area can be just as off putting as too few. Take the following example for instance:

You’re exploring a dungeon an RPG when you suddenly realize that the only thing keeping the thirty goblins in the first room from being eaten by the dragon in the second room is a flimsy wooden door. Sure the door denotes some kind of boundary, but still, such coincidental proximity rarely occurs in nature. Why doesn’t the dragon just break down the door to snack on the goblins who would make easy prey? This mistake is fairly consistent throughout game worlds: creatures that do not belong together are placed together simply because the designer wanted to switch up the tempo of the encounters… a commendable goal, but one that is too often achieved at the expense of believability.

Ultimately, Chex Mix style population makes for an exciting game world, a world where anything can be waiting around the next bend, but it also induces a kind of mindlessness in the player, a state in which the player enters a new level and immediately goes through the same laundry list of tactics that worked in the first few environments. It destabilizes the game world and forces the player to suspend disbelief, both of which are detrimental to creating an immersive experience.

So, how then does a designer know where to draw the line between variety and believability of encounter types? Simple, just apply the litmus test of asking yourself if the creatures that you’re mixing together exist in a hierarchical relationship.

Take the above example of the goblins and the dragon: what if rather then placing the dragon in the room next to the goblins, the dragon was instead confined to a lower cavern of the cave-system, tucked away in a secluded passage, its lair littered with goblin skulls? Certainly portrays a clearer idea of how the goblins and the dragon interact with one another, does it not? Not only is this set up more compelling from a world building standpoint, but it also justifies the dragon’s existence in the dungeon in the first place; it’s there to eat! It also obliquely explains why all the goblins have banded together in the upper cavern: they don’t want to be eaten! (This pair of facts ties in nicely to the rule of congregating creatures in groups around resources above). This makes sense because the goblins and the dragon exhibit a predator-prey relationship. They interact with each other as well as the player.

There are many hierarchies that exist in the real world. Just to name a few there are hierarchies of predator-prey relationships, parasite-host relationships, master-slave relationships, leader-follower relationships, creator-created relationships, parent-offspring relationships and so on.

Half Life's small yet menacing head crabs

Here’s the deal, place as much encounter variety into one area as you like—I’m not going to impose any rules here—all I ask is that for each NPC type you use, make sure it has a hierarchical relationship with at least one, but preferably all, other NPC types in the immediate vicinity. Going back to Half Life as an example, remember the head crabs? They’re these fast little buggers that leap out of nowhere and try to latch onto your face. When the head crabs are nearby, you also encounter their victims, which are humans with head crabs latched to their faces, the parasite obviously controlling and altering the DNA of its host to turn them into a hulking zombie.

The head crab and the head crab victim are two very different encounter types from a game play standpoint—one is small and fast and agile and the other is slow and hits like a truck—but they make sense together because they have a clearly defined hierarchical relationship.

So when designing your next game level, throw some artificer mages in with your clanking iron golems. Path a Predator hunting party through your Alien-filled canyon (assuming you have access to both the Predator and Alien movie copyrights!). Spawn goblin slaves around the ogre slave master. This will cinch your game world together by conveying how all the creatures relate to one another. They’re not just waiting around to fight the player; they have their own lives, activities and organizations going on.

  4) Tying It All Together

Create consistent ecological patterns for players to learn

So, what is the end result of the three guidelines provided above? Ultimately, if you tie creatures visually to their environment, congregate them around resources, and imply that they co-exist within a hierarchical relationship with other creatures, you are weaving the disparate elements of your game world together into one believable whole, which will ultimately mimic the real world—a mess of different elements connected via ecology.

That’s not all though. These world building guidelines—assuming they are properly implemented—will spill over into and enhance game play. If you prescribe to the notion that games are essentially about learning and mastering patterns, and I believe most game designers share this assumption, then the ecology of your game world will be another pattern for your players to learn, a pattern that bridges and reinforces the sub-patterns provided your individual encounters on a creature by creature basis.

To understand how these guidelines can enhance game play, take the examples used earlier in this article. When you, the savvy player, encounter head crabs in Half Life, you get your shotgun ready, because you know you are going to need something with stopping power to take down any of the lumbering head crab victims that may be lurking nearby.

Similarly, in World of Warcraft when you see the chitin towers in the distance, you know that you will soon be fighting silithids, and so you get your area-of-effect attacks ready for the numerous, but fragile, larva that the silithids tend to spawn. And, in the imaginary example of the goblins and the dragons, you as a player are ready for a big fight when you begin to see goblin bones littering a cavern you have just tiptoed into. Ultimately, the three guidelines laid out by this article can be seen as the connective tissue that holds a game world together. The tighter and thicker this connective tissue becomes between, the more believable your game world will feel, and the easier it will be to reach what should be the ultimate goal of all game designers, specifically stellar game play that works on multiple levels.

Sony's PlayStation 2 action epic God of War

Remember, over half of the video games available on the market today take place in worlds different from our own. Whether a player descends towards the forlorn planet surface of Zebes (Metroid) or hacks a path through the monster-ridden lands of Ancient Greece (God of War), most games offer a creative escape, a departure from the world of everyday life in favor of a new imaginary setting. Some game worlds are downright alien. Others are hauntingly familiar. Regardless, the best of these worlds are the ones that mimic the patterns and of our world, while still remaining internally consistent with the assumptions, no matter how far-fetched, of the game universe itself. Only then can a player begin to learn and respond to a game’s setting as a dynamic and believable system.

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About the Author(s)

Kevin Carter


Kevin Carter is finishing his graduate studies at UCSD. Prior to returning to school, he designed adventure modules for Wizards of the Coast's Dungeon Magazine and expansion titles for Sony Online's EverQuest, including the latter game's tutorial and demo (available as a free download: www.escapetonorrath.com). He recently accepted a summer Product Marketing internship with Electronic Arts, about which he is very excited.

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