Defender by any other name ... is just as difficult! Cosmic Raiders runs slowly and not particularly smoothly. The spaceship looks like it's running on half cylinders. Firing the craft's laser freezes the entire screen for a brief period. Despite all this, the game is good fun to play and will bring you back to the keyboard for another try.
As with all derivatives of the arcade massacre, the aim is to preserve stupid humans from the hovering alien kidnappers. They are not making it easy for you, however, as they have all climbed to the top of high buildings and are waving about frantically. Even the most short-sighted alien couldn't miss this lot.
Still, save them you must against landers, mutants, mother ships and the rest. If things get too frustrating, shooting humans is easier than shooting aliens, and often more satisfying!