Each stage begins with 5 rows of 11 aliens in formation.
Fire upwards between (or through) the shields to destroy the aliens. Destroy each alien to advance to the next stage.
You must avoid getting hit by bullets fired at you by the aliens.
The four shields above the surface of the moon absorb shots, both yours and the alien's.
You can create a hole in your own shield to fire up through, but alien bullets may drop through the hole as well.
As you destroy more aliens, the remaining formation speeds up.
The last alien will move the fastest, moving faster while going to the left than to the right.
To prevent the aliens from dropping down sooner, concentrate on the aliens positioned in either column edge.
If the player's bullet collides with an alien's bullet, one bullet will cancel the other out, and it can be either your bullet or the alien's that continues on.
A saucer randomly appears above the formation which you can try to hit for bonus points.
Every time you complete a stage and start a new one, the alien formation begins one row lower than before, up to a certain minimum height above the surface.
If an alien's path intersects with a shield, that section of the shield is removed.
To get 300 every time you hit the mystery saucer. It is named the 'Furrer Trick' named after Eric Furrer who perfected it. To do this trick, perform the following :
Once the new level begins, start counting the shots from your base.
When you have fired 22 shots, stop shooting and wait for the mystery saucer.
Use the 23rd shot to blast the mystery saucer.
After this, start counting shots again, this time count only to 14.
Use the 15th shot to hit the mystery saucer.
Continue using the 14 shot rule until the level is finished.
At the next wave, start with the 22 shot rule, then use the 14 shot rule to finish that wave.
Remember, all shots count regardless of hits or misses.
