3D Defender is a 1st person shooter where you have to save your planet from alien invaders by blasting and destroying them. You fly over the surface of your planet searching for the aliens with the aid of a radar which shows you any ships in close proximity. As you blast the aliens and attempt to avoid their lasers you must be careful not to hit the surface as this loses you one of 10 shields but you have an altimeter telling you your distance from the surface. Getting hit by a laser or an alien ship also loses you one of your shields. Destroying an alien ship gets you 250 points but letting them land loses you 50 points but you can get these back if you destroy them when they take-off again.