Increasing mana cost on skills is bad. I casually upgrade things and find myself more than once unable to use a skill because suddenly its mana cost is way higher than my base mana. You can tell me now that I need to make "clever tactical decisions" before upgrading, but overall this just feels "meh" from a game design point of view.
Not trying to bash this nice game, I just think you should look into ever increasing mana costs. Next ascension I will leave some auto-use skills on very low levels, just because I don't feel the bonus in the skill is worth the exploding mana costs. Is this really your intention?
The mana increment is something that I wasn't sure how to solve - I'm not sure which stage in the game you are at, but I've received comments from end game players where mana is overflowing (esp on the thief and warrior) and there is nothing to do with the excess MP. Making the skill mana costs linear would amplify this problem. Sadly I still haven't got a clean solution to the issue of mana, where early game there is a shortage of it and in late game it overflows.
The amount of coins needed for one gem does not reset after a soft reset. This makes collecting gems increasingly harder and come to a halt after some time. I would recommend a reset to the inital coin/gem value after a soft reset.
I just lost most of my units on the first campaign castle. Now I have 0 units again, that feels like I unintentionally reseted the game! How about a warning that I kill my own units in campaign?
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The mana increment is something that I wasn't sure how to solve - I'm not sure which stage in the game you are at, but I've received comments from end game players where mana is overflowing (esp on the thief and warrior) and there is nothing to do with the excess MP. Making the skill mana costs linear would amplify this problem. Sadly I still haven't got a clean solution to the issue of mana, where early game there is a shortage of it and in late game it overflows.