It's free to play, but you cannot use certain functions of your city if you don't pay. Actually it'll end up costing you a bit if you really want to play as you'll need cards to start your army. (You get some default ones but they're weaker than the ones you can buy) It's a fun game but try it out before you buy anything. I was a beta tester for the game so I ended up getting alot of starting stuff for free.
