Throughout the last year, Supercell's fortress defense game "Clash of Clans" and King Digital's "Candy Crush Saga" vied for the title of highest grossing free-to-play mobile game. This year, "Clash of Clans" has been decisively dominant.
At first, it might simply seem like a battle between two tremendously successful outliers. But the outcome may indicate what type of game will entice users to spend money in the future.
The market for so-called midcore games like "Clash of Clans" is growing as gamers seek a deeper experience from free-to-play titles, say analysts. To be sure, casual games like "Candy Crush Saga" remain immensely lucrative. But mobile developers are discovering that more immersive game play increases engagement—and engaged gamers spend more money on virtual goods.
The distinction between casual and midcore games can look fuzzy at first, but it matters. Research firm Newzoo estimates that global revenue for more challenging midcore games accounted for $3.8 billion of the total $14.9 billion mobile market last year. That's nearly double the $2 billion in revenue they had the previous year.
"It's starting to get to the point where, if nothing else, you have to have a little bit more of an enticing offering, a larger narrative and broader brand," said Joost van Dreunen, CEO of SuperData Research.
The little bit more that midcore titles offer might be a storyline, high production value and customizable characters—elements that make "core" console and PC games immersive. Midcore mobile games pair these elements with simple, swipe-and-tap controls. They're less repetitive than puzzle games like "Candy Crush Saga," and often incorporate real-time strategy and role-playing.
As casual gamers graduate to these games, they are increasing their spending, say analysts. In the first two months of the year, "Clash of Clans" saw average revenue per user increase from $1.29 to $1.34, according to SuperData Research. Meanwhile, average spending among "Candy Crush" players remained unchanged at just under 60 cents during the same period.
Other top earners include "Modern War," a military-themed role-playing game from Funzio. Strategy titles "Game of War" from Machine Zone and Kabam's "The Hobbit: Kingdoms of Middle-earth" took the number 5 and 11 spots in the iOS app store in March, respectively.
Analysts say these games owe their success to the developers' ability to boil down the experience of classic real-time PC strategy games such as "Warcraft" and "Age of Empires" for touch-screen play.
"Companies that know how to adapt their core title or theme to the mobile platform—the casual aspect that a mobile platform forces on you—they are the most successful," said Peter Warman, CEO of Newzoo.
Primary targets for midcore developers are 30-something fathers, many of whom stop playing expensive PC and console games because they can no longer dedicate the many hours they take to master. Members of this demographic also tend to be employed and have disposable income.
Warman said Newzoo regularly takes meetings with developers who want to know what sort of game play and intellectual property will appeal to these lapsed gamers. The audience might be valuable for another reason, though. According to a Newzoo survey, about 70 percent of fathers say they spend most of their mobile gaming money on their kids. Warman suspects that these dads influence their kids' gaming habits.
"There's a double story. If you can convince him to play on iPad, he might suggest to his kids to play that game," said Warman.
Some first-generation social games are now incorporating strategies that make midcore games successful.
Zynga, the beleaguered company behind the "FarmVille" franchise, is one of them. The company, which was slow to migrate to mobile, has seen its stock tumble more than 50 percent from its 2011 IPO.
Zynga took a step toward midcore gaming by improving production value in 2012 with "FarmVille 2." Now it is rethinking game play with "FarmVille: 2: Country Escape," its first game designed specifically for smartphones and tablets.
Morgan Stanley recently improved its outlook on Zynga's stock based on its opinion that new game mechanics for "FarmVille" better align its ability to make money and user engagement.
For example, Zynga shifted away from a model in which users either complete repetitive tasks to advance or pay to skip ahead. That game play worked when gamers plowed their digital fields on the desktop, but the note says mobile users often reject it.
The new "FarmVille 2" promotes engagement through competition and gives users more options, like buying premium items with credit earned through game play, rather than money alone.
Still, Morgan Stanley warns that competition in the mobile gaming market makes it difficult to predict success.
"Even if the titles in Zynga's mobile pipeline are much higher-quality than its previous launches and / or competing games, there is still no guarantee that they generate material bookings," said the note's authors.
With more titles entering app stores every day, that warning could apply to developers of all stripes.
—By CNBC's Tom DiChristopher.