A block of fifty aliens, five rows of ten, marches sideways across the top of the screen and drops twenty pixels closer each time it reaches a wall. You control a ship at the bottom that slides left and right and fires upward. Clear the block to advance a wave. The run ends if you lose all three lives, or if any alien descends to the height of your ship.
Aliens pay more the higher up they sit. The back row is worth 50 points, then 40, 30 and 20 coming forward, and the front row nearest you is worth 10. There is no wave bonus, so your total is just the sum of what you shot. You start with three lives and lose one to each enemy shot that connects. Each wave sets alien speed to 0.5 plus 0.2 per wave and enemy bullets to 4 plus 0.3 per wave.
Firing is limited to one shot per twenty frames. Bullets already in flight keep travelling, but the next one waits for the cooldown.
If any surviving alien reaches the vertical position of your ship, the run ends at once regardless of lives remaining.
Yes — on a touch screen the game shows on-screen buttons, and games played with a mouse work by tapping.