Good, but could be better
Definitely engaging as a tower defense/strategy game. However, at the point I'm at currently, the variety of things seems a bit superfluous. Equippable items always add a little extra something, but in the end simple stat changes aren't as exciting as different kinds of equippable weapons could be. The variety of mercenaries you could hire is likewise interesting, but also seems a bit unnecessary once you've accumulated mercenaries of all the different elemental types. There's a variety of enemies with different resistances... ...but again, this too seems like it could somehow be boosted. Perhaps the graphics of the actual gameplay sequences don't seem as sharp as all the menu and quest-choosing screens.
All in all, it's a good tower defense with some good points (I liked that the enemies cycled back to the beginning even after passing the end-point and taking off a life; in so many other tower defenses, I get so mad when I ALMOST kill a boss and they wander off-screen. Here, I've got the chance to finish 'em and make them run my gauntlet of towers again!), but for the amount of effort that's gone into it it seems like there could be something more to it.