Free-to-play mobile games are difficult to develop under the best of circumstances, and Aeria Games' "X-Men: Battle of the Atom" video game certainly does its best to rise to the challenge. Announced last year and meant to coincide with the "Battle of the Atom" event, "X-Men: Battle of the Atom" debuted this week after a period of beta testing in Australia, and though the game and its mechanics are incredible confusing, once understood, the game is surprisingly addictive.

Much like the iOS game "Marvel: War of Heroes," "X-Men: Battle of the Atom" is a card battler, meaning that players recruit characters in the form of cards and use them to battle evil. The core concept of "X-Men: Battle of the Atom" is that the X-MenNew Mutants, the dimension-hopping Exiles or even Alpha Flight, there's a diverse amount of characters to choose from. Cards are classified under a certain rarity depending on their power level and availability. There's common, uncommon, rare and high rare, with high rare being the most powerful cards. Players get one free "recruitment" each day, and each chapter of the single-player story has three different cards that enemies can drop. To guarantee higher rarity cards, players will have to pony up some real-world cash or play through the story.

Players use 10 character cards in their mission squad, which allows them to mix and match to create their own customized team of X-Men. This is where knowledge of the X-Men universe and relationships come into play -- for example, Gambit and Rogue will not only get a bonus from both being on the X-Men's Blue Team, but they also get a personal relationship bonus, both of which will increase their attack and defense power. For those that aren't as familiar with X-Men trivia and relationships, it's also possible to gain a bonus by playing cards that share a power classification (Mental, Energy or Physiology), which is printed right on the card.

Perhaps most satisfying about the game is its ability to fuse two of the same cards together to make a more powerful version of the same card. Called "Evolution," it seems to be the key to getting higher-rarity cards. Cards also have the ability to level up through "Transfusion," a way to get rid of unwanted cards to make your squad more powerful. This mechanic isn't exactly original, but it's used in many iOS card games for a reason: it works well and is fairly intuitive once you understand it.

The game's single player missions are where "X-Men: Battle of the Atom" really makes an impression. The pixellated versions of the Jean Grey School's senior staff and mobs of enemies -- from Hellfire goons to Savage Land dinosaurs -- are charmingly old-school, and the usage of art from the actual comics goes a long way. Players familiar with modern X-Men history (from "Schism" on) will find a lot of familiarity here, especially fans of "Wolverine and the X-Men."

There are a few limiting factors in "X-Men: Battle of the Atom" that encourage players to take on the story campaign. Stamina allows players to take actions, Squad Points increase the number of more powerful cards that an X-Squad can accommodate and Command Points are used for player-vs-player encounters. When a player levels up, points can be put into Stamina or Squad Points, but it can be slow going. Only 3 points are available to distribute per level.

Squad Points are what makes the game a little more fair for those that choose not to purchase 10 high rare cards right at the start of the game, as High Rares use up more squad points that the set of commons and uncommons players start with -- and since all X-Squads must consist of ten cards, it's going to take a lot of single player campaign grinding before you're able to use that high rare. Players certainly can pony up real-world cash to grab some High Rares, but it's going to cost them a pretty penny. A guaranteed High Rare starts at 300 X-Coins (roughly $3), which is a pretty steep price tag for a single card that you might not even need. Plus, there's no guarantee you'll be able to use it until you have enough squad points.

There is a multiplayer component known as "XvX" battles where players have the option of forming X-Teams with other players to take on teams across the globe. It's an interesting concept, and the game really pushes players to take part with special currencies awarded to those that participate. X-Teams are also ranked, so competitive PvP players might find quite a bit of depth to the system. Another key part of the game, Trading, is currently not available.

While Aeria Games has clearly spent a lot of time and effort on the story and art of the game, the tutorials are going to be a bit confusing to those that aren't familiar with these types of games. As someone very familiar with this type of mobile game, I struggled to figure out how XvX battles worked and how to efficiently build X-Squads. Additionally, there are a limited number of battle animation effects that seem to show up with little or no predictability. I've seen wind effects hit enemies when a fire character was in play, or Gambit's charged playing cards when it was clearly Dust from the New X-Men.

The game also suffers a bit from a confusing and unintuitive user interface. Trying to find a specific card among 50 on a tiny iOS screen is absolutely maddening when attempting to transfuse, and the ability to sort alphabetically isn't an option. To bring up a larger version of the card, users have to tap and hold on the specific card, which is completely unintuitive. That said, these are minor flaws and can be easily fixed in subsequent updates.

All in all, "X-Men: Battle of the Atom" is a much more polished, better realized version of a Marvel card battler than "Marvel: War of Heroes." The game is all around better thought out, more focused and -- given the chance -- extremely addictive. Time will tell if the game ends up having legs, but there's a lot to like if you're an X-Men fan.

"X-Men: Battle of the Atom" is available now on the iOS app store.