Lots of potential, but fatally flawed.
Pros: The spell system, with its upgrades, was nice.
The save system was helpful.
Cons: Insanely difficult. If it is genuinely diffcult to beat level five of a defense game, there's something probably something wrong. Slow the growth of the baddies, and particularly the monsters with regen. It takes me nine spell charges from lightly upgraded spells to destroy a level 2 Trino, or more if I don't fire off the spells as quickly as I am able to do so.
The Strength/Weakness system needs to be tweaked, perhaps by making the monsters take fifty percent less damage from spells of their element, rather than simply ignoring them.
The multiple paths for the monsters didn't really appeal to me. The screen got too crowded, too quickly, too often. Either reduce the rate at which the monsters appear or eliminate a path or two.
On the spell upgrade/purchase screen, show the player the stats on new spells before they are bought.
As other reviewers have noted, you should allow the player to expel all remaining spell charges from the selected spell, ideally with the space bar.
Also as other reviewers have noted, there should be a heal/repair option. In that vein, you might allow the player to upgrade the maximum health of the building.
The story appeared too slowly. Either make it appear on the screen all at once, or speed it up.
The grammar errors were mildly annoying, but not a huge issue.
Graphics: Not terrible, but definitely amateurish.
Sound: The music and sound effects fit the game well.
Overall: 6. A decent game with lots of potential for a future version. It's fun for the short amound of time that it lasts.