In 2012, something unexpected happened: a Japanese-developed collectible card game became a sensation in the West. Rage of Bahamut, developed in Tokyo by Cygames and published by Japanese mobile giant DeNA's U.S. subsidiary Ngmoco, became one of the year's top-grossing games on both Android and iOS.

Bahamut is an unqualified hit -- so much so that it established the genre, which already reigned on Asian phones, as a full-blown phenomenon in the West. Just as importantly, it went a long way toward proving the widely held supposition that niche-targeted mobile games can monetize reliably off of a smaller, more engaged audience.

When it comes to the game's success, says Akihiro Iino, company director of Rage of Bahamut developer Cygames, "We weren't surprised, because there was already a market overseas for card games. However, we also did not foresee the whirlwind of popularity that followed."

In fact, nobody anticipated that "whirlwind of popularity." It's clear, however, that since Rage of Bahamut hit the App Store and Google Play a year ago, the genre has become a durable facet of the mobile ecosystem. As of this week, Rage of Bahamut is still the number three grossing app on Google Play, alongside other card games like Marvel: War of Heroes (number two, also by Cygames) and Dark Summoner.

A year is a long time, but things can -- and will -- change. Is the genre itself a fad, or is it here to stay? What does it take to make a successful collectible card game? To find out more, Gamasutra spoke to the companies who know: the Rage of Bahamut partners, Cygames, DeNA, and Ngmoco, as well as DeNA's chief rival, Gree, and Japan's biggest success in the genre, Konami.

What's the appeal? "This game [genre] is considered to have high potential from user retention and monetization aspects," says Kimihiro Horiuchi, senior director of Konami's Dragon Collection Studio San Francisco, who's tasked with making Japan's biggest card game hit, Dragon Collection, a U.S. success. His words sum up the simple truth of the genre's explosion, post-Bahamut. But is there more to the story than that?

Unpacking the Popularity of Card Games

How did a game like Rage of Bahamut -- one even some of the staff at Ngmoco even initially refused to try, according to its former CEO -- become a smash?

"I can't say that we knew that it was going to become one of the most successful mobile games in history," admits Doug Scott, vice president of Marketing & Revenue at Ngmoco.

But on reflection, says Scott, the success makes sense: "Collectible card games are an incredibly elegant framework for great mobile social gaming experiences. As a vehicle for game design, they tap into many of the core compulsions that have driven great games throughout history: strategic resource collection and application, and competing with your friends in a battle of wits and skill."

DeNA's rival is Tokyo-based Gree, and its U.S. SVP of social games, Eiji Araki, does a fantastic job of explaining what appeal the genre holds for players:

"This genre of game plays directly into the player's desire to gather the ultimate set of cards," Araki says. "Among CCG fans, there's a strong sense of accomplishment when a new or rare card is collected, making for addictive gameplay that keeps players coming back day after day."

"Another reason for its popularity," Araki continues, "is the social engagement elements baked into the gameplay mechanics, such as cooperating with other players to trade cards or battling other players in PVP modes to win cards to further their collection... Similar to physical card collection games that have been globally popular for years -- like Magic: The Gathering, Pokémon, Yu-Gi-Oh -- mobile CCGs go beyond being 'just another mobile game' to players."

In fact, Cygames pays particular attention to the appeal of the cards, says Iino. "Making sure that our artwork is peerless in quality, we also pay attention to the smallest details that might not be obvious. We believe that this is one of the fundamental aspects of game design."

The games are popular because of their compulsive, collector-driven gameplay, of course, but Horiuchi also argues that it's their service-based nature that is the key to their success.

"We have continued our service improvement while observing user behaviors constantly. It's been two years since Dragon Collection started, and we think this constant improvement is another reason to gain support from many users for a long term." Marketing alone won't build success in the genre, he says; continued development will.