When he was a boy, growing up in Nottingham, England, Andrew Gower couldn't afford to buy all of the video games he wanted to play. Rather than mope, he rallied. A wunderkind programmer, Gower created his own versions of the most popular games, pieced together from clues printed in text and image in the pages of video game magazines. Gower's take on Lemmings—the 1991 Amiga game that was developed by DMA Design six years before the studio made Grand Theft Auto—was his masterwork. "I was proud of that game," he says. "It was the first [computer game] I’d made that didn't look like it had been put together by a kid."

Gower would grow up to become, along with his brothers Paul and Ian, the co-founders of Jagex Games Studio and creators of its flagship title RuneScape. It's one of the longest-running massively-multiplayer online games (MMOG), in which players quest together across the Internet in a fantasy world that, like Facebook, continues to rumble and function even when an individual logs off.

Launched in 2001, the earliest version of the game looked rather like a fantasy-themed version of The Sims. Characters were viewed from a divine camera, looking down on the action from an isometric perspective. RuneScape takes place in the world of Gielinor, where gods roam among men. The game eschews a linear storyline, allowing players to set their own goals and objectives. Now in its third iteration (the basic game was superseded by a new version in both 2004 and 2013, each of which upgraded its graphics and overhauled the underlying code base), RuneScape has reached an enviable milestone in the fickle world of MMOs: 15 years old.

To mark the notable birthday, RuneScape's developers have made a documentary film for the occasion, the rather plainly titled, RuneScape - 15 Years of Adventure. The grandiose stat-waving that introduces the film should be eyed, as with any piece of rosy propaganda, with a pinch of scepticism. It's true that 245 million registered accounts puts the game's population at more than three times that of Great Britain (a figure that also places RuneScape as the fifth largest country in the world, by inhabitants). But this fact, so eagerly recounted in the documentary, is only a theoretical population. There are usually fewer than 100,000 players online at any given time.

By comparison RuneScape's rival, World of Warcraft, accounted for 62 percent of the global subscription-based video game market in 2008. Blizzard no longer discloses the number of active subscribers to its game, which has been haemorrhaging players for years now, but the most recent official figure from last year is still a substantial 5.5 million. By contrast, the most recent official figure for RuneScape's paying subscribers, published nearly a decade ago, was around a million.

One accolade, however, cannot be questioned: RuneScape has endured for more than a decade where few massively multiplayer online games survive. Since the first 3D virtual worlds launched in the mid-1990s, more than 50 have closed as a result of declining populations. Some, such as Lego Universe, lasted just two years (it closed in 2012). That game had their servers switched off without any ceremony. Other developers ensured a proper send off for their beloved world, making the termination a formal conclusion to the game's fiction.

When Star Wars Galaxies closed in 2011, for example, the developers set off an extravagant firework display, providing a sense of occasion for the game's lingering population that stuck with it through its eight tumultuous years. RuneScape has endured in one form or another since 2001. Originally a free download at a time when the elbow-y term "free-to-play" was yet to be coined, it grew quickly and sustained its audience with weekly updates. It once earned a Guinness World Record for "most-updated game," in fact.

In one sense, the RuneScape documentary is a kind of high-production corporate video, outlining the company's history and focusing on glowing testimonials from apparently contented employees and dewy-eyed customers. "The community in RuneScape is perhaps the warmest community in gaming," says one. Initially, it's not entirely clear who the film has been made for. But watching the Gower boys' parents, Gill and Chris, reminisce about their children's early love of Dungeons & Dragons, hold up Paul's crayon drawings of castles and knights (who went on adventures that tallied with the family's excursions) to the camera, and recall Andrew's obvious early talent for programming—"He disliked being interrupted so much that he invented a program that would shout out 'Intruder alert' when anyone went into the room"—it's easy to become swept up in the nostalgia.