Turtle Rock Studios' Left 4 Dead is one of the best online co-op games in the past decade, and the studio's upcoming four vs. one game, Evolve is shaping up to be another hit. One of the keys to Turtle Rock's success is its emphasis on rapid prototyping -- find the fun first and iterate and build from there. "The game is what is today because we've been playing it for the past three years," said Turtle Rock creative director and cofounder Phil Robb in an E3 interview. "The basic idea is to get the game up and running as soon as possible in its crudest possible form," he said. "Not that we intend it to be crude, but you use what you've got. The nice thing about working in the CryEngine is that it's real easy to make prototype stuff look pretty." For example, he said when prototyping the first Left 4 Dead, the studio just used terrorist and counter-terrorist skins from Counter-Strike, just playing around with the mechanics, design and enemy behavior. He said Turtle Rock narrows down the core elements of the game. For Evolve, it was about designing for four vs. one, co-op vs. single, and having a monster that evolves and becomes more powerful over time. That's the frame of the game that was in place when prototyping began. The result is a game that pits four hunters, each of a specific class and skillset, against a monster that becomes more powerful over time, across three evolutionary stages. "The hunters are sort of even all the way across [Robb traces a horizontal line in the air], but the monster starts low [on the graph], then crosses that line," he explained. "Stage one he's lower, stage two he's even, stage three he's got a bit of a numbers advantage. Those are things we tried to get in asap." Robb added, "The whole idea behind the rapid prototype is that we don't want to spend a lot of money on something and not know if it's fun. "I've known tons of people, and I have even been on games where you couldn't play the game until the end. That seems like a huge, crazy risk to me. You'll see a lot of games that came out that clearly suffered from that. It's like, 'wow, they should've been playing this from the beginning.' "So that's what we try to avoid at all costs -- we get it up and make sure that it's fun, and then start iterating and building aspects to it. You add the depth as you go."

Turtle Rock is open to Early Access for future titles

One of the most notable trends in the video game business is the rising popularity of selling games as alphas and betas. Steam Early Access has been a key factor in this trend. Asked if Turtle Rock would like to release an Early Access game at some point, Robb replied, "Dude, I would love to do that, and Chris [Ashton, cofounder and design director] feels the same way. Who knows, I won't say 'never,' but I can't say 'yes' for sure. Robb said it would've been fun to break down that barrier between developer and player, letting their community take part in and inform the way the game is developed and designed. "Even on [Evolve], as we were playing it all these years, the game changed a lot, and there'd be features that'd go in, features that'd come out. We always kind of lamented the fact that we couldn't take the community along on that ride. "On Left 4 Dead it was the same thing," he added. "We had weird Infected in the game that we ended up yanking out -- we'd have fun with them for maybe a month or so, but ultimately they weren't right for the game. But even then, we think about those instances, remembering how they were messed up, but fun to play around with." And that's what's appealing to Robb about Early Access -- enjoyment of the game transcends the game itself. Of course, changes to an Early Access game -- like removing, adding or tweaking features -- might annoy or upset players, but that'd be part of the deal when taking part in development of a game. "For us [when playing the game internally], that always keeps it fresh always keeps it new. 'Hey we're trying this, let's check it out.'"