Description: Earth in the middle with the player controlling some sort of gun to shoot/destroy incoming threats at the edges of the screen (asteroids, etc) [the cannon you could rotate around earth and was closely attached to it], weapons could be upgraded I recall (rockets, homing rockets, bigger explosions, etc)
Game used to have both a demo version and a full version , you played it in multiple levels/stages
Year of release: Tough to say, I played it around 2002-2007 I think.
Graphics is a bit fuzzy .. it seemed more than 2d but less than 3d
Further info:
No plot or story, just trying to keep anything from hitting earth in the middle.
Perspective is flat, earth in middle, threats slowly approaching from the sides
Only objective was to stay alive and complete each stage, you'd get better weapon upgrades as game progressed.