Galaga was a more powerful version of Galaxian, an earlier spaceship shooting game made by Namco.
In the NES version, you control a spaceship that goes side to side. You try to kill the insect-like enemies that keep gathering to attack by firing missiles at them or using them as weapons. This game is different from others because the aliens tend to attack as a group. Even though this can sometimes make their paths clear, most of the time the player will be moving quickly from place to place to try to avoid the blaze.
Even though you might want to just keep hitting the fire button, this will hurt your score because you get a bonus for accuracy at the end of each level. Since the game has an infinite number of rounds, the accuracy bonuses become very important if you want to get the top score.
Have fun and good luck!
