It's been years since I've played this game. I have good, fond memories of playing it for hours. After having not played it for so long, I thought that surely it couldn't be as fun as I remembered. It turns out yes, yes it can.
Placement of towers, deciding which upgrade paths to follow for which situations, what skills to take either in general or for a particular level, when to cast spells, which spells to cast, all of these decisions add a ton of challenge to an otherwise simple tower defense game.
Whilst finishing each level on its own isn't usually difficult, getting three stars on both the day, and especially the night levels, adds a whole layer to the game. Sometimes just placing towers differently helps. Sometimes you need to save your mana for a clutch fear. Sometimes, you just need to come back after another level-up, or rearrange your skill tree.
The only reason this isn't a full five stars is because after playing for a long time, it either lags terribly or bugs out (Night levels can have the light indicators not show, the upgrade menu for towers not clicking the first time or closing after upgrading, or on one occasion just loading a blank grey screen save for the gem cave, which was inexplicably at the top left. I've been unable to consistently recreate the errors though). A restart always fixes both problems when they happen.
All in all, best tower defense game I've played.