The game is linked to the theme by the fact that during the 15 minutes the waves of enemies increase, and to survive you have to improve your spells accordingly, making them grow (e.g. increase radius, etc.). It's more related to the "evolution of your spells" than anything. I do admit that I could have worked a bit more on how the theme is linked to all that.
And thanks for the feedback! Glad you liked the game!
Edit : forgot to respond to you for the whole "regen linked to leveling" part : The issue is that magic regen is OP and it's what make the separation between being completely broken or being weak. Making it a part of the leveling may be a bit too OP I think. But I'd have to try to be sure.