Scramble was the first ever side-scrolling shoot ’em up, coming out in arcades in 1981.
It was certainly one of the earliest colour video games, and was a big hit for developer Konami and distributor Stern Electronics.
The basic idea is to fly your ship down a series of caverns, shooting rockets that rise vertically, and aliens that fly in front of you. It gets progressively harder, as these things do, and you have to keep your fuel topped-up by destroying fuel tanks with bombs.
Having two different fire buttons (one for bullets, out the front, and one for bombs, down below) was also quite innovative for the time.
Scramble still provides a few thrills – and a fair challenge – to any retro-gaming fan who loads it up now.
More: Scramble on Wikipedia