'Batman: Arkham City': How the Dark Knight went open-world

As you may have already experienced following this week's launch, Batman: Arkham City is something very special indeed. While game sequels often turn out better than their predecessors, topping a BAFTA-award winning title in Batman: Arkham Asylum - a game that practically appeared from nowhere and surprised everyone with its atmosphere, action and attention to the rich Batman lore - is surely no easy feat. While there are many areas this sequel improves upon, we were particularly interested in seeing how it accomplished its biggest and most radical transition - going from a linear adventure to dense open-world - so well.

"The number one thing [we wanted to do with Batman: Arkham City] was take the game into Gotham City, Batman's spiritual home," Rocksteady Studios's game director Sefton Hill told Digital Spy. "That's what got us excited, and that's where I think our best ideas come from. If we're looking at something, and we're excited, then that's going to translate into the game.

"A lot of the other stuff came out of that. If he's in Gotham City, having this walled-off district of Gotham allows us to bring in these supervillains, and [we can see] how they react with each other and with Batman himself. It allows us to think about how Batman moves through the city, now it's much bigger than Asylum, and developing the whole movement and traversal mechanics." It's those three core advances - the wider environment, how Batman explores them with new moves and the roster of villains, that were the centre points of where Rocksteady took the sequel.

But that wasn't all. Batman: Arkham Asylum was famed for its density and rich environments, something that Rocksteady wanted to replicate in the follow-up. "We also wanted the same level of detail as Arkham Asylum. Our goal was never to create the biggest open-world ever, it was to make a condensed, really rich [one], where every square inch of this game has a purpose and a value, or tells a story," Hill revealed. "I felt that there wasn't a game that did that; we had big games, linear story games, but not something that had the feel of a linear story game but in an open environment. So that was our goal."

Despite such an ambitious task set from the start, the final size of the city remained almost the same from concept to completion - there was around a 5% to 10% difference. Even then, the team also did a lot of experiments to ensure it was comfortably explorable. "The size we spent a lot of time on, because it has to feel like it's something that you want to transverse across, not like it's too time-consuming," said Hill. "If you want to go to the other side, it can't be like, 'I can't be bothered to do this, it takes too long'. The size of the city is something we messed around with a lot."

Having a larger city while retaining that same density and attention to detail was a challenge felt across all departments, especially when it came to art and atmosphere. Art lead David Hugo explained that it wasn't just the sheer work involved, but it was ensuring that story references were carried throughout. "I think that was interesting, when we talk about [increasing] details of stuff, it's not the type of details where there's a pile of bricks there and a pile of rubbish there, where it's normal mapped textures and stuff," he said. "It's the detail in terms of references to the Batman universe and elements that we can grab and work around the city, a lot of different things that attach themselves into the city. Those were the challenges."

Having a sense of personality and variety in the city was made easier through the villains. Characters such as Mr. Freeze, Two Face and others all have their own areas in the game that Batman must explore through the course of the game, each with their own feeling and themed enemies. "We sliced the world up in different ways, or into different districts," said Hugo. The Joker's lair, for example, combined the series's signature clash look of "hyper realism" and stylised feel of the comics, by having a traditional warehouse filled with funfair elements, from lights to big clowns and Ferris Wheels. "Bringing these two elements brings this visual explosion, something interesting to look at, and refers to the character, which is interesting," added Hugo.

In terms of work load for the studio, all departments bumped up their numbers for the sequel, but they stayed reasonably small in order to get the job done effectively. 3D animator lead Adam Vincent said that the studio didn't like to outsource, as Rocksteady preferred everything done in-house. "[Batman] is as important to us as it is to DC, to have the franchise," he said. It was added that his department, which was bumped up from 64 people for Batman: Arkham Asylum to 105 for Arkham City, was a comfortable enough number where he could easily know everyone on a personal level and manage the team effectively.

Outside of villain layers, an open-world isn't quite complete without enemies to bump into and scrap with along the way, and taking Batman: Arkham Asylum's enclosed encounters into a much larger arena was one of the game's biggest challenges: "The AI now have to know that Batman can come at them at at any direction and go at any time," said lead AI coder Tom Hanagin. "In the first game, you enter a room and fight the guys, and that's how it works. Now, you can sneak up behind a thug, pick 'em off from one side, everyone will attack you, but then you can glide away and go all the way around the room and come back in a completely different direction.

"We never had to cope with that before, so from a technical AI point of view, we have to make sure they have to do things that are cool, and when you sneak up on them, you can try and misdirect them by moving around or using gadgets from unseen locations, or throwing Batarangs at them from around corners. I think that was the technical challenge." Of course, the open-world layout also provides another different design challenge - what's the incentive to fight enemies when you can just evade them entirely over rooftops and backstreets? Again, it's that rich attention to detail that pulls players back in.

"A lot of what we do is entice the player to sit and listen to the thugs and realise that most of the thugs have something interesting or funny about the story to play," said Hanagin. "But you don't have to take them on at all, you can fight past most of them. That's your decision as Batman, you can make that decision if you want to." Of course, there are two other obvious benefits to fighting foes on the streets, with experience points to help you prepare for future encounters and the sheer satisfaction of fighting, with a free-flowing combat system that builds upon the many successes of the first game.

That decision to see the series leap from claustrophobic corridors and enclosed halls to an entire city district has undoubtedly proved a challenging one for the team, but across the board it appears that the rich Batman mythology, which the writers and designers have adapted into their own dark, twisted and unique universe, has kept them focused on creating a place that's a joy to explore. And after incorporating so many characters and places so successfully, where can the series go next? Rocksteady are understandably tight-lipped, but with the team now proving themselves to be one of the UK's strongest development talents, you know it's going to be just as stand-out.

Batman: Arkham City is available now on Xbox 360 and PlayStation 3, with a PC version releasing next month.

> Read our five-star review of Batman: Arkham City