On the surface, Undead Labs' State of Decay feels like a conventional open world game. The zombie apocalypse title is set for an XBLA release some time in 2013, and it features both an interface and third-person perspective familiar to anyone who played the likes of Grand Theft Auto, Crackdown, or Saints Row.

Surface-level differences between State of Decay and these other titles become readily apparent just a few minutes into a recent demo. Chief among them is the ability to fortify buildings into something “similar to a GTA safe house, but more robust” in the words of John Gronquist, one of the artists on the project. You have to fill in the limited number of build locations in each safe house with elements like gardens, sleeping areas, medical tents, and workshops, adding a real time strategy vibe to the proceedings. Players aren't tied to a single protagonist in State of Decay, either. When a character dies, the player takes control of one of the other survivors in their community to continue playing.

While those are important distinctions between State of Decay and other open world games, the most significant change might lie under the hood. During the demo, Gronquist referred to the game as having been “over-engineered.” He said Undead Labs performed an “extreme amount of engineering into the procedural simulation of the world... We wanted the world to be this organic thing where everything you did had a consequence to it.”

What does that mean, exactly? Undead Labs founder Jeff Strain and Creative Director James Phinney explained the world of State of Decay stays dynamic even when the player isn't watching.

“Instead of the world always being in a static state and always waiting for you to change the action, change the state of things, we said 'OK, what if we’re just simulating the state of the world and things are progressing?'” Phinney told Ars. When a player gathers survivors to join their community, for example, those survivors don’t just stand around while the player goes off to search for resources. “They’re out doing things all the time, too, trying to take care of the community, following the higher-level direction you set. And as we started to explore this feeling of having the world be alive and active, we just started to commit to it more and more.”

So unlike most open-world games, the world of State of Decay isn't just a backdrop for the narrative missions, but a simulation that is constantly progressing and changing. “Events are going on with or without you, and you choose how and whether to interact with them and that will modify the state of world, and if you ignore it that will also modify the state of the world,” said Strain.

One of the game's zombie types, called a Screamer, exemplifies this dynamism. Screamers have no arms and no lower jaw, so they can’t grab or bite the player, but their moans and screams can attract other zombies. Simulating natural Screamer behavior eventually and inevitably leads the Screamers to attract a huge zombie horde around them.

In other words, choosing whether or not to dispatch a Screamer now can have an effect on how many other zombies gather in that area later. Keeping a Screamer alive can turn previously safe areas quite dangerous, leading troubled survivors to call the player for help, requiring diversion of important resources. If State of Decay were constructed as a traditional single-player game, events like these might be hard-coded into the larger narrative. Instead, the game allows these kinds of events to emerge naturally from the simulation.

“[In] open world games, generally there’s this really strong distinction between the sense of freedom you have causing mayhem, and then the spots where there’s real content,” said Phinney. “There’s some storyline stuff that’s going to happen in a specific spot [in State of Decay], but what we did was try to as much as possible, and it is the majority I think of your play experience, have stuff be really heavily driven by what you’re encountering in the world and the state that you have left the world in.”

“Just a straight-up, more traditional open world zombie survival game would have been awesome, and would have been possible without this stuff,” Phinney continued. “But what we’ve found is, by going down this road, it’s a different experience entirely.”

And, again, that simulation doesn't stop when you stop playing. “When you stop playing the game for the day, conceptually the world is still running,” Strain said. “When you start up the game, what we effectively do is look at the amount of time that’s elapsed and forward the simulation by that amount of time. We’re careful not to punish you for that. You’re not gonna log back in after two weeks and find your bases destroyed and all your people are sick, but as a conceptual design platform we do say that time goes on, even when you’re not there.”

The kind of world a player returns to in a new State of Decay play session will depend largely on how the player’s behaved in the last play session. If you made sure your people had growing food and ammunition workshops and medical facilities in their safe house, they’ll be in good shape when you log back in. If you didn't plan for the future, though, you might find your survivors have gotten into an untenable situation.

Despite being what Gronquist called an “over-engineered” game, State of Decay seems to have just the right amount of engineering to create an open world game with some life to it. “Certainly we’re not doing anything gratuitously,” Phinney said when asked about this distinction. “We’re doing this because we had a vision of what this kind of experience could be, and we’re putting in all the core pieces to make that happen.”

“To address that point directly, I will say that it is over-engineered,” added Strain. “I mean, you are right, everything we’ve done has been expressly for the purpose of creating this living, breathing world…however, had our goal from the beginning been strictly to create an open-world sandbox zombie survival game, then there certainly would have been a much shorter and more direct path than the one we took. When we say it’s over-engineered, you hear us use this word 'simulation' all the time, the simulation itself is very robust, and far more robust than we would have needed to make it just to pull off that direct statement of the type of game we wanted to make right now.”

Undead Labs plans to supplement the single-player State of Decay with a massively-multiplayer experience in the same universe, code-named Class 4 (State of Decay was previously known as Class 3). This companion game helped motivate the creation of the robust simulation.

“We very much have been of a mind that with everything we’re doing in State of Decay to look at how it sets us up, how it teaches us, how we put ourselves in a better position to make a more perfect online world game,” said Phinney. “And that is, I think, exactly what Gronquist was talking about. There’s a depth to some of the systems that, sure, if we thought of it in terms of 'We’re trying to get people X many hours as a single-player game,' we wouldn't have needed to go into quite that level of depth, but because we’re looking at building systems that will extend [into Class 4], the side benefit is, as we’re working on it now, the variety we get out of the depth is really awesome.”

Edit: The original version of this article misspelled James Phinney's name. Ars Technica regrets the error.