Invasion Force is a fixed screen shoot 'em up where the player, as so often before, has to defend the earth from an army of hostile aliens. The player moves his laser base along the bottom of the screen. At the top there's a large enemy ship that is protected by a moving force field. In addition to this there are smaller ships that drops bombs towards the player. The objective of each level is simply to destroy the large ship, but to be able to reach and damage it the player first has to shoot holes in the force field. Hitting the force field causes the missiles to ricochet back towards the player. The force field also has black parts that should be avoided, since hitting them will fully repair the enemy ship. To take down the enemy ship the player has to hit the control area located in the centre of the ship. The game is over when the player has lost all of his three ships or when the time runs out.

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