The goal of the game is to blast the alien forces and get the highest score possible with your three lives.
The tougher the enemy is to destroy, the greater the chance they will drop a valuable power-up.
Collect them to help fight off the alien vermin and earn yourself the highest score.
The power-ups can be a one-shot event or a timed event.
There are FOUR different powerful items to collect, Triple Shot, Single Beam Shot, Shield and Slow Time
The alien Command Ship will randomly spawn from time to time and hover across the screen behind the alien invaders.
If you destroy the Command ship, you’ll be awarded a random point value between 100 and 500 points.
The difficulty ramps up quickly with the infestation’s relentless attack intensifying as time progresses.

More details on the official game page of Alien Invasion ...