Finnish gaming company Supercell is a $3 billion company largely in part to the success of its game Clash of Clans.

Clash of Clans is a strategy game that is free to play, but some people end up paying hundreds of dollars on in-app purchases and upgrades.

So it's no wonder why Clash of Clans is among the top 10 grossing apps in the iTunes Store, according to AppData.

So what's the appeal behind Clash of Clans?