The first non-Xbox Halo game has been released in the form of Halo: Spartan Assault. It can be purchased only on Windows 8 and Windows Phone 8 (the PC version of the game is available here), and while we couldn't find the listing on the UK WP8 store, the PC version will set you back around £4.59. Controls are either touch-based, or with mouse and keyboard, depending which format you grab, and there's 25 missions to play through, weekly challenges, and XP earned can contribute to your Halo 4 profile.
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