Galactic Battle (1990)

This Space Invaders-style shooter pits the player against eleven different waves of alien opponents which recur over the course of an infinite number of levels. The player can move horizontally and (to a limited degree) vertically, and the traditional gun is accompanied by a shield that can be activated at the press of a button. Everything costs energy, however, so there is a time constraint to play. Medals are earned during play which can be redeemed for ship enhancements (reduced energy use, increased fire power or increased shot damage) if the player is able to successfully dock with a starbase. Docking also refills the player's energy after each wave.