This game is 100% free to play, but if you put money into the game you can get things faster, sometimes a lot faster.
Heroes that are game changing like: Goram, Sylphi, Orkon and Vulko are very hard to get without spending money, unless you are very lucky and get them from the Crucible of Fire chests (I was able to get Sylphi there) or to summon them from the Diamond Chests. All this heroes will be in the shops eventually, but it may take a couple of months until that happen.
About leveling up, it is possible to reach level 100 without spending any money, just takes you a few more months.
So, this game is free to play, but you will have a big advantage if you pay sometimes, because the best heroes will be available much faster to the players who pay for them.
If you like my posts add me:
Click: Avatar -> Invite Friends -> Invitation Codes -> Enter Code