An old school shooter that has a top-down / isometric perspective, similar to the one that characterises many hack 'n' slash RPGs. But Alien Shooter is not an RPG. It's all about the mindless action. Notice the adjective mindless! You press down your left mouse button and you do not stop until all aliens are dead. At the same time, of course, you try to get away from the smaller creatures and avoid the projectiles of the bigger ones.
Although the game has some RPG elements (you can visit stores regularly and there's levelling up) main emphasis is on the action. It's a shame that SIGMA made this game very short and that they recycled most of the elements in the game that later became known as Zombie Shooter part 1 and part 2. I assume the game's next installment, Conscription, looks and plays exactly like this one so I did not bother purchasing it.
Still, you can pick up this one or one of the Zombie Shooter versions and have a couple of hours of fun when SIGMA games are on sale.