The game featured a relentless march of invading aliens with an ever increasing tempo. You were in the tempest in the fury of survival, with diminishing defenses. Your only recourse was a high score in the name of planetary defense.
As the player, you moved a mobile ground based canon that shot (one at a time) into the sky towards the descending invaders. The invaders marched in cadence, back and forth across the screen, descending after every cycle. The cadence increased the long the aliens persisted. The best way to play was to make every shot count, and to make the shots connect with the lower rows of aliens first. The aliens would shoot back, and you had several bunkers to hide behind. Though the bunkers were damages by the aliens' blasts.
A very memorable part of space invaders was the increasing tempo as the invaders drew closer. This usually had the effect of driving the defenders into a panic, resulting in folly and a landing of the aliens!