The canon can be moved horizontally by the player pressing the left or right arrow keys. The player can fire bullets from the canon by pressing the space bar key. Only one bullet from the canon may be in play at any time. If a bullet hits the bottom of a bunker it breaks away some of the bottom of the bunker and then another bullet may be fired. If the bullet hits an alien then the alien explodes and another bullet may be fired. If the bullet hits the mother ship then the mother ship explodes and another bullet may be fired. If the bullet does not hit anything, the player must wait for the bullet to exit the top of the screen before another bullet can be fired.
The aliens randomly drop bombs from the lowest row of aliens. There are two different types of alien bombs. There may be more than one alien bomb dropped at any time, but usually no more than four will be dropped together. The bunkers are designed to allow the canon to hide from the alien bombs. If an alien bomb hits a bunker, pieces of the bunker are blown away, but the bunker may still allow the canon to hide beneath it until too much of the bunker is blown away, letting the bombs pass through to hit the cannon. If an alien bomb hits the canon then the canon immediately explodes. If there are lives left then a new canon will appear at the far left side of the frame. The game is over when the last canon explodes or when the aliens reach the bunker row.
If the cannon hits all of the aliens in all five rows before the aliens blow up the cannon or reach the level of the bunkers then another life is earned. A new set of aliens is then displayed, starting one row below where the previous set of aliens started. Eventually, if several rounds are won, the aliens start so low that there is only enough time for them to move across the screen once before dropping into the bunker level, causing that round to end.