The free-to-play, or freemium, gaming model has exploded in the last five years, with more and more developers relying on microtransactions or in-game advertising instead of the traditional up-front payment. It's a model that was mostly explored by South Korean developers that were known for rolling out a wide variety of massive multiplayer online games.

Attempts by Western studios to adopt the free-to-play model have been mixed, and finding the right balance between in-game payments and gameplay can be difficult. But one studio that nailed it from the very start was Wargaming.net, developers of the World of Tanks MMO.

Based in Belarus, the company originally cut its teeth on turn-based and real-time strategy games. "For three years we go nothing but jokes - everybody was making a joke about us. 'An MMO with tanks? It's not going to work," Victor Kisly, the chief executive of Wargaming, said in an interview with Joystiq.

Despite the initial derision, however, the game's popularity quickly grew. In September 2010, less than 12 months after the game's official Russian release and more than half a year before its official release in North America and Europe, Wargaming announced more than 700,000 users around the world had registered a World of Tanks account, with 500,000 total active users.

The game has since entered the Guinness World Book of Records, setting the record for the "most players online simultaneously on one MMO server" with 91,311 active players. More than 3 million players had signed up before the end of May 2011. Wargaming then blew their Guinness world record out of the water late last year, with more than 500,000 concurrent players on a single server in Russia.

To date, more than 55 million players around the world have registered a World of Tanks account, a staggering achievement. Around 1500 people work at the company, which has offices in Minsk, Kiev, Paris, the United States, Singapore, Saint-Petersburg and Sydney.

So what is it that makes World of Tanks so popular?

One aspect of the game's popularity stems from its target market: the military enthusiast. While not a hardcore tank simulator by any means, the game's focus on realism where appropriate has won over a lot of fans disenfranchised with other genres.

"Who needs another elf game if there are already 500 on the market," Kislyi told Bloomberg's Businessweek last year. "We decided to go niche. You cannot go more niche than World War II."

With World War II comes an incredibly rich vein of information. There's a staggering amount of history in WoT, with hundreds of playable tanks from the Chinese, British, French, Americans, Germans and Russians.

But perhaps the game's strongest aspect is the balance between the simplicity of the controls versus its tactical complexity. Any first-person shooter fan will be able to jump into a World of Tanks game and control the movement and weaponry within minutes, but it takes far longer to learn the nuances of the game's damage system, the strengths and weaknesses of each tank type and the teamwork necessary to win.

It's a game that's carved out a rather unique fanbase, and it's one that Kislyi believes is unlikely to leave any time soon for the next big thing, even for games with as large a following as League of Legends. "World of Tanks is more for older players who aren't that agile or that quick on the clicking. They appreciate a slower, more thoughtful, more strategic game with more historically accurate visuals," he explained.

"We've [League of Legends and World of Tanks] been around together for a couple of years now. We don't see any big migrations from us to them or in the other direction. They have their niche of millions worldwide, and we have our own niche, which is 55 million registered users. We're pretty happy to be there."

Alex Walker is the regular gaming columnist for ABC Tech + Games. You can follow him on Twitter at @thedippaeffect.