The first game Jeremy Parish ever bought when he was a kid was Metroid for the NES. After hearing about the game from friends, Parish — now a writer and editor who maintains the classic gaming site Retronauts — went in expecting to experience something like a sci-fi Super Mario Bros. But that changed not long after he started playing.

“I kept going right and right and right, with no end-of-level break in sight,” he explains. “And then I kept going up and up and up. And still, the first stage didn't end. It took me a while to cotton to the continuous, exploratory nature of the game, and I spent weeks just wandering around trying to figure out what to do.

“Eventually, the game went from feeling like torment in some unknowable, bewildering, mysterious labyrinth to a familiar journey through a beloved place.”

Metroid, which debuted in 1986, would go on to spawn one of Nintendo’s most-revered franchises. The ongoing adventures of bounty hunter Samus Aran differed quite a bit from the company’s other big names, like Zelda and Mario. In comparison, Metroid was dark and solemn, with a looming feeling of isolation and a powerfully alien sense of place, inspired in large part by the first Alien film. It was also a game that felt unique in its structure. While Metroid was a 2D, side-scrolling game, it took place in an expansive, interconnected world. Players could explore in a nonlinear fashion, and would often have to return to areas using newfound abilities.

The game went on to spawn a number of beloved follow-ups, including the sublime Super Metroid in 1994, and the Metroid Prime spinoff series that transformed the 2D adventures into a first-person, 3D experience. Most recently, Nintendo is set to release Metroid: Samus Returns on the Nintendo 3DS, the first traditional side-scrolling Metroid in nearly a decade. But the importance of Metroid can be seen in more than the games released by Nintendo. The series has also had a profound influence on gaming as a whole, inspiring a generation of designers along the way.

Perhaps the biggest game to come from the mold of Metroid is 1997’s Castlevania: Symphony of the Night. With that release, director Koji Igarashi invigorated the long-running gothic action series by introducing a Metroid-like structure, in which players were forced to navigate a massive castle, unlocking new abilities to open up more areas to explore. Its impact was so profound that the style of game has since been dubbed “Metroidvania,” in honor of its two most significant progenitors. Igarashi followed Symphony of the Night with a handful of other similarly structured Castlevania games, and has even returned to the genre again with his upcoming game Bloodstained: Ritual of the Night.

“It's an experience that feels more natural.”

But it wasn’t until the rise of indie gaming years later that “Metroidvanias” really took off. In 2004, a freeware game called Cave Storywas released on PC. The game — which has since been ported to a number of platforms, most recently the Nintendo Switch — was created entirely by one developer, Daisuke “Pixel” Amaya. With its moody sense of style and freeform exploration, Cave Story’s influences were clear. “More than anything else, I love Metroid,” Amaya said in a 2010 interview. Soon, an influx of indie games would follow.

One of the more notable Metroid-style indie releases is Axiom Verge, which debuted on the PS4 in 2015, and will be coming to the Nintendo Switch next month. Created entirely by developer Tom Happ, Axiom Verge had a similarly mysterious sci-fi aesthetic, but differentiated itself with a slew of unique tools and abilities, including a gun that lets you create glitches in the world around you.

For Happ, the decision to make this style of game was an easy one once he settled on the goal of crafting a side-scrolling experience. “It comes down to a decision of whether you want separate stages or a continuous world,” he says. “The ‘Metroidvania’ style, with its ability to go backwards as well as forwards, is really the side-scrolling equivalent to an open-world 3D game like Tomb Raider, Horizon Zero Dawn, or Batman. It's an experience that feels more natural, while still providing enough structure to guide players and tell a coherent story.”

Like Parish, Happ played the original Metroid when he was young — he actually borrowed a copy from a friend in third grade — and he became immediately entranced by its world, bustling with secrets to uncover. “The fact that there were rooms outside of the main game experience made it seem very mysterious and perhaps mystical,” he says.

“There is a nice conversation a designer can have with a player.”

Other games took a somewhat less obvious approach, fusing the Metroid structure with different themes and game elements. 2011’s Guacamelee, for instance, offered an interconnected world to explore, but also introduced more complex, brawler-style combat and a lighthearted, meme-filled tone. “I feel there is a nice conversation a designer can have with a player when it comes to the layout of a game in a “Metroidvania,” Chris McQuinn, a designer on the game at Drinkbox Studios explains. “There are so many opportunities to create clever nods and winks with level design decisions that a personal connection can me made with players.”

For the creators of Ori and the Blind Forest, first released in 2015, the appeal of Metroid’s structure was the ability to create a convincing sense of place. “A ‘Metroidvania’ forces you to create a connected, believable world that you can invite your audience into,” explains Thomas Mahler, co-founder of Ori developer Moon Studios. “You don’t get too far if your designs are too abstracted, so you’re really forced to create something more memorable, because players remembering the levels is part of the core design.” Ori also differentiated itself with lush, beautiful, painted levels that felt almost like an animated film come to life. A sequel is currently in the works for Xbox One and PC.

Meanwhile, with the SteamWorld Dig series — the second of which comes out on September 21st — the team wanted to keep the sense of loneliness inherent to Metroid, but expand on it with concepts like platforming and digging for treasure. “With SteamWorld Dig 2especially, we want to expand on the feeling of isolation and create a sense of discovery,” explains Julius Guldbog, from developer Image & Form. “It really goes well with the underground setting; most of the time you're on your own and you never know what you'll find if you chip away a few more rocks. Creating a huge world is easy, filling it with enough variety is the real hurdle.”

But the influence of Metroid isn’t limited to the game’s that emulate it so directly. A number of larger games also employ a similar structure, but translate it to three dimensions. The Dark Souls series and the Arkham line of Batman games are perhaps the two best examples, as both feature a large open world, with areas that can only be accessed once you’ve found the right item or unlocked a certain ability. “First there was Zelda, then there was Metroid, and a lot of the things those two series started have influenced literally hundreds of games in the past and still influence games being made to this day,” Mahler says.

The question is where the genre goes from here. For a long time, fans yearned for “Metroidvanias” largely because so few were actually being made. That’s no longer the case. Plenty of talented indie creators have stepped in to fill the void, and now even Nintendo has returned to the franchise with Samus Returns, as well the upcoming Metroid Prime 4 on the Nintendo Switch. Parish believes that for the genre to continue to grow, it needs to continue to approach the concept with fresh ideas. “We need more games that use Super Metroid as a jumping-off point, not as a rigid template,” he explains.

For his part, Guldbog believes there’s still plenty of life in the “Metroidvania” format, even as it becomes increasingly crowded. “For me, it's a timeless style of games,” he says. “You can easily approach it from different angles and create something unique.”