A fun game but with a serious problem with progression and gems.
You actively get punished for progressing well through levels as you don't get as many gems as you would if you kept failing, and there is no gem bonus for getting a higher health score. Late in the game I actually reached a point (around day 20 I believe) that felt unwinnable due to the slowness of the progression and how much I was getting.The first mission to introduce the green worms earlier VERY NEARLY ended my game earlier than that.
The mana boost and red gem upgrades are critical.
Also for new players who want to win:
Make sure to grab the cleric and ranged units ASAP. He makes it so you have something resembling a stable foundation for defense. Completely ignore "ball" as it is near useless aside from a very few specific scenarios and actively a resource drain.