The XBLA title, which usually retails for 800 Microsoft Points, is free as part of Microsoft's 'Games with Gold' scheme.
The rewards service, which launched with Fable 3, was officially announced at this year's E3 gaming expo.
According to Microsoft's Larry Hryb, Halo 3 and Assassin's Creed 2, which were widely expected to make up July's content, will be available for free at a later date.
FAQ: "Halo 3" and "Assassin's Creed II" were mentioned as part of the program - no date was given for their release http://t.co/vjnE1sBAYj
ā Larry Hryb (@majornelson) June 30, 2013
New games will be available for free on the 1st and 16th of each month.
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Released in 2009, Defence Grid: The Awakening is a tower defence game in which players must defend against an alien invasion.
Developed by Hidden Path Entertainment, the game is also available on PC and Mac.
Watch a trailer for Defence Grid: The Awakening below:
