In Guardian, players assume control of the well-armed starship Guardian, defending a row of planets at the bottom of the screen from a constant onslaught of weapons launched by a mothership hovering across the top of the screen. The game is simple: the mothership's weapons will blast holes in a shield protecting the planets, or, if there's already a hole there, those weapons will destroy one of the planets. Some of the weapons are more powerful than others, and will blow a bigger hole in the shield. The player is able to pick off incoming fire as it rains down, but eventually there's too much to repel. The game ends when all of the player's planets have been destroyed by enemy weapons.
An interesting variation on the basic slide-and-shoot formula, Apollo's Guardian did very little to distinguish itself from its much more visually interesting and exciting contemporaries, such as Demon Attack or even Space Invaders, and the notion of protecting little planets that were almost dwarfed by the player's own ship was a bit silly. The play mechanic of repelling incoming fire was carried off much better in Attack of the Timelord! for the Odyssey 2.