Hearthstone has been a hit on PC and more recently on iPad. The free-to-play model often leaves a bad taste in players' mouths, but production director Jason Chayes says the studio worked hard to make sure real-money purchases were voluntary, not inevitable.

"We really wanted to make sure that it wasn't just lip service to say it's free-to-play and then hit some kind of monetization gate where you had to buy things," Chayes told Eurogamer. "So for us making sure you could actually earn all the things you needed to be competitive was something we put a lot of time into."

He said that seeing people construct powerful decks for free during the beta period let them know they were on the right track. "So overall we felt very good about how that evolved and how in some ways helped validate our approach, and we feel pretty good about the direction that's been heading," he said.

Chayes also pointed out that the guiding principle is creating a game that they would like to take home and play themselves. "And because of this there's a lot of internal discussion about things like pricing and how to we make sure there's good value for when I do choose to buy something, or that I can actually progress as a free-to-play player. All of these are things we have a lot of debate internally about and I feel like it's this crucible where we're trying to understand what's the best implementation we can have, and burn away some of the impurities there to get to a point where we're really happy about it."