Defender came out in 1980. Famously it was the first arcade style game produced by the pinball machine makers 'Williams". It was a hugely popular sideways scroller that looked years ahead of its time when released due to its fast smooth scrolling. Game play is simple enough defend you astronauts from being abducted by the aliens by wiping them out first. Take too long and the said aliens start to carry your men up and away requiring you to shoot the alien (not the astronaut!) and then swoop in to catch the falling dude. Sounds simple now but this was a game that would have your heart rate soaring until it got too much for you and you had to press the hyperspace button. As with asteroids a few years earlier the hyperspace button needed an element of luck as you had no control over where you would reappeer.
Defender ruled as the king of sideways scrolling games until the following year when Konami released Scramble. This game sees you 'Jet' fly across the continuously scrolling landscape while waves of aliens come at you and missiles are fired from the ground. later levels there is an incredibly difficult meteor shower to navigate that I never managed to get past. To make matters worse you had limited fuel which needed to be replenished by bombing the fuel tanks on the ground. More experienced players soon developed the skill of hugging the ground and mashing the bomb and missile button as fast as possible. Thus avoiding a lot of the dangers. These new features brought a new depth to simple shooter type games.