Answer: Defender

It’s around forty years old now and there’s a good chance you may have never even heard of it, but 1981’s Defender is considered one of the most important titles of the golden age of video arcade games.

The 2D space shooter, designed by Eugene Jarvis for Williams Electronics, was the first arcade game to feature a side-scrolling environment such that the player appeared to be advancing through a given level/virtual space as they played. What’s remarkable about the game is that it was Jarvis’ first foray into video game development (his prior experience was in pinball machine design).

Thanks to the popularity of the game—it sold over 55,000 units—millions of arcade goers across North America got their first taste of side-scrolling action. Although rudimentary in delivery, the concept was ground breaking and heavily influenced the later development of similar-genre games like Gorf and the general industry.