Galaxy Defenders is a sci-fi cooperative tactical combat game where 1-5 players fight together against an approaching alien threat. Each player takes control of one or more agents with unique powers to defend the planet from alien invasion. The gameplay revolves around a tactical combat system, using custom ten-sided dice. Each player successively plays their Agent turn and then one Aliens turn. Players take their turns, agent and aliens, clockwise until the last player completes their Aliens turn. When this is done, the game moves to the event phase which takes the players to the next round. Players can choose up to five agents: Marine, Biotech, Infiltrator, Sniper or Hulk.

The battle for Earth is carried out in a series of twelve missions organized into a fully story-driven campaign. Mission events affect future games in two different ways:

1 - Each mission has multiple endings and the outcome of each mission will change the flow of the story.

2 - The agents gain experience during the missions. This experience transforms a good soldier into a perfect Galaxy Defender agent with multiple skills, basic and enhanced tactics and the ability to use new devices, enhanced human weapons and Alien technology.

The downloadable Galaxy Defenders: Alien Mind variant lets you transform the game into a competitive affair, with one player becoming the alien mastermind and controlling the alien army and maps in the game, trying to thwart the agents' every mission. To do this, the alien player completes their own game objectives and obtains new "alien signals" that can be teleported to the battlefield. This variant, which allows play with up to six players, can be used in a single mission or for an entire campaign of Galaxy Defenders. Using Alien Mind can increase the difficulty of the game and is only recommended for experienced players.