Defender is one of the many space games back in the 80's thanks to the popularity of Star Wars and Star Trek.
The game was built by Eugene Jarvis, who wanted to build something different then the simple games of then such as Pong, Space Invaders, & Pac-Man. The game was originally suposed to be a version of Space Invaders & Galaxian where the player would move sideways but, eventually changed to a game where the player would move in all directions.
Defender is a very complex game for it's time. The game had waves of aliens called "Manti" come down to the surface of the planet to attack and try to abduct the human colonists so they could feed on the human DNA and become more powerful mutants. The player would try to destroy the Manti landers on their way down by firing the pulse gun on the defender ship. If the Manti had captured a human then the human could be recued by blasting the alien and flying into the colonist. However, if the person didn't get to the human in time...
....well they'd have a mutant lander on their hands. The Manti can also shoot.
The game had two sequels one in the 80's another in 2002.