I'm really surprised how much I'm enjoying this, I started playing a few months ago and I've been playing daily ever since.
It's a simple combat system that works extremely well for touch based controls.
The thing that has hooked me into this game, is the progression/leveling/collectables, in most free to play games, you tend to hit a wall where progression just stops, I've been playing this game long enough that I should have hit that point by now if it was coming.
I think this game gets the free to play model right, there is no need to play if you are satisfied to play 15-30 minutes a day, if you want to play more, pay.
The one issue I had with this game is the system requirements, you need a fairly highend phone/tablet to have a good experience with this game, it looks and plays great on my Nexus 9, it didn't run very well on my older tablet though.
This is a very good free game.