INVADERS as you have probably already discovered is a great deal of fun, but it is only played on one plane - vertical. ASTEROIDS is more "three dimensional", but it is concerned with inanimate objects, namely lumps of space rock. With DEFEND you actually feel as if you are piloting the craft. The result is a game with a high addictive content. Your objective is to shoot down a squadron of aggressive enemy ships. If you are skilful or lucky enough to obliterate them all, you have to navigate through a shower of meteorites. If you get through that then you must navigate a tunnel before you are in the clear. You then automatically return to the beginning and begin another orbit, albeit at a somewhat harder level.