You must destroy every alien in the formation to advance to the next wave. You will lose one life if struck by any alien bullets. If you allow one alien to touch the ground, the game will be over regardless of lives. The aliens move across the screen in one direction. When one alien reaches the edge of the screen, they will drop closer to the surface and move in the opposite direction.
The more aliens you destroy, the faster the remaining formation will move. You have four shields spread out over the surface that absorb shots, but erode over time. A saucer will occasionally fly over the formation, which can be shot for bonus points.
- Left/Right buttons: Use to direct the cannon along the bottom of the screen. Push left to move the ship to the left, and right to move the ship to the right. Your cannon can not pass beyond the side of the screen.
- Fire button: Press the fire button to send one missile directly above your position. You can only send one missile out at a time, but you have an unlimited supply of them.
- 1 or 2 Players: Push these buttons to begin a one or two player game.
You control the cannon. Use it to destroy the space invaders' formation. Direct the cannon left and right to choose a position to fire from. Time your shots so that they arrive at the location of an invader that you are trying to hit. Use the shields to protect the cannon from enemy fire. Be aware that both your fire and the enemy fire can chip away at the shields. Getting hit by an invader's bullet will result in the loss of one cannon. An extra cannon is typically awarded at 1500 points.
The three alien invaders may look different, but they each behave the same. The remain in formation as they advance towards the surface of the moon. The entire formation will move to the right or to the left until the far column reaches the side of the screen. Then the formation moves down one row and returns to the opposite side of the screen. The lowest invader of each column will fire bullets down to the ground. If any one invader manages to touch down on the surface of the moon, the game is over, regardless of how many lives the player has in stock.
The saucer appears randomly over the formation. It appears on one side of the screen and crosses over to the opposite side. During this time, you can attempt to shoot it for bonus points. The point value that is awarded appears to be random, but it can be controlled to some extent by counting your shots (see below).
- Large Invader: 10 points
- Medium Invader: 20 points
- Small Invader: 30 points
- Saucer: 50 ~ 300 points