The game includes: one spaceship, four rows of invaders with 9 invaders in each row, spaceship bullets, invader bullets.

Press the spacae bar to fire the bullets from the spaceship.

Spaceship bullets move vertically (bottom to top) at a steady speed. When the spaceship reaches the edge of the scene then the spaceship stops moving until the user changes the spaceships direction and then the spaceship begins to move again.

At every clock tick a number of invaders get to fire bullets.

Every time an invader is hit by a space bullet the score should increase by 3 points.

The game allows for 3 lives.

All space invaders move down every n clock ticks.

The game ends under the following conditions: The player has lost all 3 lives, or, All space invaders have been killed, or, The lowest invader on the scene has reached the same y-coordinate as the spaceship