A surprisingly immersive game for its type.
The gameplay is very simple. Either alone, choosing one of the two characters, or in 2-player mode, you move through the ship, pointing and shooting at aliens while trying to rescue your comrades. You can find different types of weapons along the wall, from lasers to a flamethrower or fireballs, to change the type and intensity of the damage you do to the particular type of alien found on each ship. Once your comrades have been rescued, you can proceed to the exit and fight the boss. Yes, it may sound simple, but for what it is, it still stands the test of time as a classic that wasn't appreciated in it's day.
I've owned this game since I was a kid, and back in the day, man was it scary. And terribly difficult. It's not so scary anymore, and it's gotten considerably less difficult, but it still is challenging, and all in all, is a good atmospheric run-and-gun arcader that, in my book, is no less than a classic.