This is the illigitiment sequel to Space Invaders.

Just when you thought you pushed back those pesky invaders in the 1980's, a recent scan has revealed them near the moon trying to establish a foothold for their next invasion. Fortunately, Earth managed to beat them to the surface to construct a missile defence system.

Control the missile Defender and blast the Moon Intruders, and prevent them from reaching the surface!
Updated Description:

Added the ability to change the key map (for Zire 21 and Zire 31 users).