Foxhack: The Steam forums say this thing is infested with real-money microtransactions. So much for me buying it.
Luisfius: Isn't that just in the phone version? If it is in the PC version as well (and not ingame currency stuff) then that's such a shame.
Well, there is a "+" sign next to your gold display, and when you click it, you get to a box titled "Need more gold?", where you can choose different amounts of gold to purchase, for various amounts of Euros (for me). I can't tell if the purchase actually works because when I click one of the options, the game just displays an hourglass forever.
According to the forums, the gameplay does not require the additional gold, and all content is accessible without paying extra. Still, if it's a question of principle, one should indeed stay away from this one.