Defender

Defender is one of the original building blocks of the gaming universe. You'll have undoubtedly come across it in one form or another, as it's been released on just about every format available, and spawned a thousand imitations.
Born in the era of Pac-Man and Space Invaders, Defender broke the mould by allowing the game screen to move left and right, with the game world eventually looping around on itself. Playing as a small space ship, the player has the ability to fire a forward laser, and drop a smart, eradicating all the enemies on the screen.

This may sound like shoot 'em up norms in this day and age - but that's all thanks to Defender.
As the title suggest, it's not all death and destruction - the strategic element of the game play leads to the 'Defender' name. Dotted along the landscape are ten humans, in need of a hero. The attacking Aliens will attempt to kidnap them by lifting them off the planet. With the use of the radar at the top of the screen, it's your job to see that doesn't happen.
The innovative element in Defender was the way in which you saved these humans. Shoot the alien carrying them away, and they'll fall back to the surface. Fall from a great height, and the fall will kill them, so you have to scoop them up with your ship. To make matters worse, they don't react well to your lasers, so your aim had to be spot on! Graphics from that era where always minimal, but then they were never the main attraction. Simple sprites helped you to focus on game play and enjoyment, and the action was always fast and furious.
Of course, this was back in the day when the main aim was always to stay alive long enough to rack up the high scores, and in that respect Defender is no different to other games of it's era : shoot the aliens, save the humans, collect the points. Although the basic game play never changes, there's an undeniable charm, and a fierce difficulty curve that keep you coming back for more. No wonder it's been seen on so many computers and consoles through the ages... and spawned so many imitators. Remember that next time you play Choplifter, Gradius or Strike Force.
Interesting Fact : Along with Pac Man, Defender is the highest grossing video game of all time. Another reason to get yourself a slice of history!