Thanks for the TWBK recommendation, downloading now.
Since you mentioned Hex-based games, let me recommend my absolute favourite mobile game. It's a very shitty-looking turn based strategy game called Hoplite.
The gameplay is simple to pick up - you descend into a dungeon, on each level (which fits onto the screen) there are an increasing amount of enemies, and one shrine where you can improve your abilities - throw your spear further, jump more squares, and so on.
Simple as it seems, there's an incredible amount of depth. You can build extremely different characters that require a different approach and tactics. After a few hours playing it feels truly chess-like in how intuitively you find yourself planning moves ahead.
It's got two modes, one is the main "campaign" mode where you can descend to level 26 to "win", or go further for bonus points. The second is a challenge mode with hundred of challenges.
I got it for £2.99 on iPhone and it's given me an embarrassingly large number of hours of enjoyable gameplay. No DLC or in-app purchases, but if this developer ever releases anything again I will be all over it like a powerful blanket.