Defense Grid: The Awakening

The story is based on alien invasion of a planet that has a dormant defense grid. As the player is re-activating the defense system with the assistance of a computer to control the machinery and with only limited resources, the aliens attempt to steal power cores. Power cores are "tiny floating orbs" that provide vital power to the defense grid. Cores are carried by aliens, and when the aliens carrying them die, the cores are dropped, and will slowly float back to their central holder. Once all power cores have been taken from the level, the game is over. Only one power core needs to remain when the last wave of aliens are defeated to complete a level.
Defense Grid is set in the distant future. Most levels feature ancient ruins with defensive military bases built amongst the decaying structures. The game was originally planned to be set amongst an urban environment which has been ravaged by war but that was changed to something more original. The main character in the game, which is called Fletcher, is a computer with human-like qualities that wants to protect his homeworld from destruction.