Hidden Path Entertainment announces plans to update Defense Grid on December 7th with an expansion called Defense Grid: You Monster, which will add Portal's GLaDOS to the game:

GLaDOS, the menacing AI from Valve’s awesome Portal franchise, guest stars in this new 8-level Defense Grid expansion. Players will battle not only alien hordes, but the deviousness of GLaDOS as she tests the player’s skills and changes all of the rules. In addition to the 8 new maps, Defense Grid: You Monster features 35 challenge missions, many new achievements and over 15 hours of additional game play. Defense Grid: You Monster will be available for $4.99 or 400 Microsoft points and requires the original game to play.

Hidden Path also announced that today the free Community Challenge update to the original Defense Grid game, has been released. This update features improved UI, over 30 new challenge missions and five new game modes, providing dozens of hours of new gameplay for free, whether players purchase the expansion or not!

Defense Grid: You Monster is just the latest chapter to Defense Grid: The Awakening. Joining previous map expansions: Borderlands and Resurgence, You Monster adds new environments and modes of play, requiring new strategies to beat back the aliens and defend humanity. Frequently cited as the best tower defense game available and one of the 25 highest rated games by users on Xbox LIVE Arcade, Defense Grid is for many the definitive tower defense experience.