Alien Invasion is a fixed, shooter arcade game for the Mac. In this game, you control a ship which is equipped with a cannon with unlimited ammo. Level after level, you have to destroy waves of enemies by shooting them down and avoiding being shot down by their weapons. You have three lives, and after that you lose.
Unlike other similar games, in Alien Invasion, you shoot from behind partial cover. In this case the cover seems to be huge concrete walls. After a while, if the enemies or you shoot at the walls too much, they will be destroyed, and you won't be able to hide behind cover any more.
The first level pits you against a group of enemy ships. In this case, they all move left and right as a group, so it is easy to shoot them down. In level two, they move independently of each other, but they move in a pattern that makes it easier for you to shoot them down. You can just sit in a single spot and shoot constantly and the level will be complete in no time.
There are power-ups and different weapons that you collect by shooting special ships. After a few levels the game gets very challenging.
One of the things that I didn't like about the game is that you have to shoot the enemies dead center. If you miss the center by a little bit, the enemy won't be destroyed, but when an enemy projectile comes anywhere near you, you die.