That gold cartridge meant something. The Legend of Zelda reached American shelves in 1987 as one of the first console games with a battery-backed save. No more passwords. Your quest was yours, sitting where you left it, in one of three slots, held there by a coin cell soldered inside the plastic.

That battery is the whole design, really. Once a game can remember what you did, it can afford to be long, and it can afford to let you wander off. Miyamoto has described the idea as a miniature garden you could poke at in any order, and the game means it: the second dungeon is reachable within a minute of starting, the fifth is optional if you know a trick, and the sword in the opening cave is technically something you can walk right past.

The vocabulary of secrets is small and used relentlessly. Bushes burn. Certain walls give way to bombs. Some rock faces move if you have the bracelet. Once you've learned those three verbs the entire overworld becomes a surface to test, and the game quietly rewards the kind of player who bombs every suspicious cliff. It shipped without a map, so we drew our own on notebook paper and marked the squares we'd already tried.

Finish it and you get the Second Quest, which is not a victory lap. Dungeons are rearranged, walls you could once walk through now need bombs, and enemies are swapped for nastier versions. It effectively doubled a game that was already enormous, using almost no extra space on the cartridge.

"It's dangerous to go alone." The sword on the cave floor. Hyrule's overworld theme, written by Koji Kondo in a hurry after his first attempt turned out to be a piece of music still under copyright.

This is the cartridge that taught a generation that a game could be a world and not just a series of screens.