Halo: Spartan Assault is a top-down twin-stick shooter for Windows 8 devices inspired by classic arcade shooters Robotron and Total Carnage.

The spin-off is a co-production between 343 Industries and Gatling Gears developer Vanguard Games, and is due out in July 2013 for $6.99 (the UK price is yet to be announced).

Spartan Assault will run on any device that runs Windows 8, including the Surface tablet, Windows 8 phones and Windows 8 PCs. Your progress is saved across multiple platforms, so you can start on Surface, for example, then continue on a Windows 8 Phone.

While the game has been optimised for touch devices you can use a mouse and keyboard if your device supports it. Xbox 360 controller support for the PC version will be added a couple of weeks after launch, 343 executive producer Dan Ayoub told Eurogamer at a recent briefing in London.

Spartan Assault uses virtual thumb sticks on either side of a touch device. The left controls movement of your Spartan, the right controls the direction of fire (there's a subtle lock-on assist to help you out). You can throw grenades, switch weapons, melee and jack ground vehicles using taps of the screen - but you can't jump, and there are no flying vehicles. The adaptive virtual thumb sticks move with you as your thumbs slide about the touch device.

The story is set between the events of Halo 3 and Halo 4 and revolves around Commander Sarah Palmer's rise from ODST to Spartan. The 25 missions are UNSC Infinity simulations of past battles against the Covenant, and Ayoub reckons it should take "upwards of two hours" to work through the entire campaign. There are multiple mission types, including protection, escort, survival and seek and destroy.

As in Halo 4, in Spartan Assault you can customise your loadout, choosing your primary and secondary weapons, and toggle skulls and boosters. In-game you earn experience points and Achievements, and you'll unlock new abilities and weapons, including some entirely new to the sci-fi series. You can also unlock exclusive emblems for Halo 4. "Looking forward you're going to be able to unlock some other things as well," Ayoub teased.

There's no multiplayer at launch - 343 will consider adding it after the game goes live - but there are leaderboards and weekly challenges designed to keep you coming back for more.

And, predictably, there are in-app purchases. You can grind to earn upgrades, or you can speed up your training, as Microsoft puts it, by buying credits for boosts, unique weapons and armour abilities with real-world money.

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Ayoub said 343 created Spartan Assault because it wanted to give players a new way to engage in the Halo franchise.

"Traditionally players have always enjoyed that first-person shooter experience, but you've more or less been tethered to your living room television set. We wanted to make Halo literally mobile. How do you take this experience with you wherever you go?

"The second goal was to create the right game for the platform we're talking about," Ayoub continued. "We looked at a lot of other franchises that have just copy and pasted their play experience onto a mobile platform. That's not what we wanted to do.

"The final goal we had for this was to represent and show off the platform. Halo has always been a vanguard for the platform. It's always set the standard for what's possible on Microsoft platforms, and we wanted this to be no exception. We wanted people to look at this game and say, 'Oh my god, how did you guys do that? That looks amazing.'"

Ayoub was evasive when we asked whether Spartan Assault will eventually launch on non-Windows 8 devices, such as the iPad. "The goal with the game has been to push Microsoft platforms," he reiterated. "So at this point all of our development efforts have been focused on Windows 8 platforms to highlight those devices."

And what about Xbox Live Arcade? "We wanted to start on touch devices because the goal was to take it out of the living room and make it mobile," Ayoud replied. "Who knows what we're going to do down the line? This is our focus for now."

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343's publishing team, which is around 10 people, has been working with the Amsterdam-based Vanguard Games on Spartan Assault. Vanguard was set up in 2010 by Martin de Ronde and Michiel Mol, co-founders of Killzone developer Guerrilla Games. "We in the publishing team have been interested in touch-based devices for some time," Ayoub said. "We've been looking at different ways to play Halo, but we wanted to take our time with it."

Meanwhile, Microsoft announced a three-part comic book series called Halo: Initiation from Dark Horse, which goes into more detail on Sarah Palmer's rise from ODST to Spartan and ties in with Spartan Assault.