Surprise! Once a free icon, indy game Dwarf Fortress finds new life on Steam

Dwarf Fortress, a quirky independent video game that has been free since its 2006 release, is now among the online marketplace Steam's best-sellers -- to the surprise of its US creators.

Set in a medieval fantasy world, Dwarf Fortress is what's known in the gaming world as a "construction and management simulation," in which players supervise a group of dwarves in building a mighty castle.

The classic, bare-bones version of the game remains available for download, but since December 6, players can buy an upgraded version with a soundtrack and better visual graphics.

Releasing the paid version on Steam has opened the door for more players to access Dwarf Fortress, and it's now the fourth best-seller of the week on the platform.

But the success has been unanticipated -- even though the creators, American brothers Tarn and Zach Adams, had an inkling of latent interest.

"We have internal metrics, so you can kinda say like, 'oh wow, this is actually gonna be pretty cool probably.' But it's not a sure thing, right? And so we're still surprised. It's awesome," Tarn Adams said.

Illness in the family created financial necessity -- and creation of the paid version.

"I got sick," said Zach Adams, a co-creator along with his brother Tarn.

"I had medical insurance, so I was able to pay for my healthcare bills, but Tarn didn't," he added.

After Tarn became hospitalized, and the typically-high US medical bills came pouring in, Zach said the duo decided it was time to monetize the pared-down text-based version with an upgrade.

"Seeing so many more people able to play it is awesome," said Tarn.

The paid version of the game, available for $29.99, has received an enthusiastic reception from the public, with 96 percent positive reviews on Steam.

"I waited 15 years to give Tarn my 30 dollars," said one reviewer. "Two guys with a dream are putting triple A companies with thousands of employees to shame," said another.

The Adams brothers developed Dwarf Fortress out of their small studio, named Bay 12 Games, which is located outside Seattle, in the northwestern US state of Washington.

The uniqueness of Dwarf Fortress lies as much in its complexity as in the richness and depth of its worlds, randomly generated and offering an almost infinite number of possible outcomes. 'Losing Is Fun' Another key aspect is that player's choices only have a marginal influence on how events unfold, while their fortresses are almost inexorably doomed to be destroyed, whether it's by cyclops, dragons or giant spiders.

"It feels a little bit like you open a history book and you're sort of in the middle of it," says New Zealand blogger Peter Tyson, author of a 2012 book about the game.

"Your interactions are important, but it's not like you're the most important person in the world."

The objective is not so much the final victory as the myriad of events that can lead to failure. The slogan "Losing is fun" has become popular among players over the years.

"When the goblins invade," Tarn explains as an example, "you pull one of these levers down here and it floods it... The goblins all drown.

"Pull a different lever -- that opens these hatches and drops them down into the goblins disposal area.

"It's completely up to the players' imagination how they want to confront the various threats and problems that arise."

Though not as well known to the general public, Dwarf Fortress has had a huge impact in the video game world, inspiring global construction and management simulation hits like Minecraft.

The game was also chosen in 2012 to be included in the permanent collection of New York's Museum of Modern Art (MoMa) alongside classics such as "Pac-Man" and "Tetris."

Though the upgraded, paid version of Dwarf Fortress may be available on the virtual storefront, the Adams brothers say they're keeping at it.

"The game is released but it's not done," said Zach.

"We're not going to just switch gears to something else. This is an ongoing project."