Most small startup iOS developers aren’t gutsy enough to take on the big dogs in the free-to-play space like Zynga, on Apple’s mobile platform. That hasn’t stopped LVL6, recently selected by YCombinator and started by one of the co-founders of install network G6Pay, to give it their best shot. So far, it’s paying off.

LVL6 has released Mercenary, Inc. on the App Store, a free-to-play title that shares some gameplay mechanics with Zynga’s Mafia Wars Shakedown, which we just covered here on Inside Mobile Apps. While they may follow similar gameplay tropes, their themes are different. Mafia Wars focuses on crime families and theft, while — true to its title — Mercenary, Inc. is based on paramilitary actions. Although presented through a simple, mostly text-based interface, the game still contains a good amount of nice-looking, hand-drawn artwork for characters and items. Three character classes are available to choose from: Heavy Arms, which heals faster; Specialist, which gains cash faster; and Marine, which gains stamina faster.

In the game, players move up the ranks of global mercenaries, taking on missions in San Francisco, L.A., and Chicago in order to earn cash, purchase new gear, and become stronger so they can take on other mercenaries. Missions require energy to embark on, which refills over time. The tougher the mission, the more energy it demands. Some missions also require players to possess certain weapons. These can be purchased in the game’s store using virtual cash. Armor and vehicles can also be bought here, and boost the player’s attack and defense stats for when they battle other players.

Players can view a list of other players open for attack, but will only be successful if they have enough soldiers and attack power. These aspects can be increased by adding other players to one’s army, an act that also boosts the player’s defensive capabilities should someone choose to attack them. A list of rivals can be kept, so that players know who has attacked them and can take retaliatory measures when they’re ready.

The in-game store also allows players to purchase diamonds, its premium currency, which can be used to quickly refill energy and get other advantages. Diamonds can also be traded for more virtual cash. The store has other options, too, such as purchasing soldiers, or using diamonds to purchase random “stolen goods.” Players can also exchange virtual currency to be healed in the game’s hospital.

LVL6, the game’s developer, was founded by three college students (or dropouts) and was just chosen for the next class of startups from Paul Graham’s incubator Y Combinator. Alex Weston Lin, who was one of the co-founders of install network G6Pay, left that company to start this one. It is based in Mountain View, California.

Mercenary, Inc. was released on the App Store on November 11 and has already risen to No. 14 among the most downloaded free iPhone and iPod touch games. It also runs on iPad. So far, the top in-app purchase has been a 60 diamond pack for $3.99, although LVL6 is offering up to 1700 diamonds for $99.99.

You can follow Mercenary, Inc.’s progress using AppData, our tracking service for mobile games and developers.