Send up anti-ballistic missiles to destroy incoming enemy fire and protect your cities.
The enemy attacks in a series of waves that may vary in the number of attacking interplanetary ballistic missiles. Each consecutive wave moves faster. The faster the wave, the more difficult it is to defend the cities. So, the faster the wave, the higher the scoring.
With each wave you have 30 ABMs for defense. Your launching missile base, (bottom, centre of playfield) contains only 10 ABMs at a time. As each set of 10 ABMs is fired, you automatically receive 10 more from you underground missile dump (bottom, left corner). Once you have fired all 30 ABMs, you are defenseless until a new wave begins.
You must protect your launching missile base from enemy fire. Once it is hit, all of its contents are destroyed. However, you still have the remaining missiles in the underground dump. The game ends when all of the cities are destroyed.