About Alien Syndrome

In 199x, when the Space Colony was first established, Aliens from the nearby planet, Alpha, invaded the Spaceships.
An urgent message has just been received from the earth, reading "rescue your captured comrades aboard the Spaceships and then blast the crafts and their Alien occupants out of the galaxy". Immediately responding to the command, you enter the first of the floating fortresses, and stand horror-struck as you see the weird and bloodcurdling array of creatures that you must confront in battle.