Enjoyable at first, in the end, only tedious. No real challenge... or even tower defense.
Where most tower defense makes you think about how best to place towers and what to upgrade when, this game is completely stripped of thought. You learn the optimal locations for each "tower" type early on, and, as evidenced by "hero mode", the levels are almost all designed with a choke or two that takes full advantage of a single group's optimal patterns, so most levels play like this:
1. Plug one, rarely two, "tower" groups into obvious optimal sockets
OPTIONAL: Spam archers
2. Upgrade (Mostly whenever, you'll have plenty of 'power'.)
The "RPG" side certainly gives math nerds something to obssess over, but it's similarly simplistic:
1. Level up!
2. Damage +1
OPTIONAL: Reset points at any time, because you somehow screwed that up
That's it. You want your healers to heal, of course, (skip Holy Light completely) and to take advantage of each "tower's" secondary effects, (armor break, chill, bleed, range, light/blind, devour) but for the most part, you'll just have to decide which usually low-end skill or two to neglect. Usually the lowest. You can safely ignore most defensive skills. (The actual numbers are utterly pointless because adequate info to calculate DPS and some secondary interactions aren't provided, and you learn how effective everything is by "feel"/sight same as any TD game anyways... but don't tell the math nerds!)
And that's the game for you. The art is bland, the story is absurd, but occaisionally amusing. Nothing to see here. While not a "worst game ever", if you're out for an actually good tower defense/game, look elsewhere. You might enjoy this if you're a math nerd, part of the Number Enlargement crowd, or just want something easy and amusing to click on and only plan to throw a few hours away with it anyways. Don't expect the game to get better as it goes on, it just doesn't. It never could, because it's fundamentally flawed.
Unfortunately, I have to add: system requirements are a trollishly bad joke.
The game runs in Flash/AIR and suffers from the usual "looks like an old DOS machine could run it/practically requires a super-computer" issues. If you don't have a video card that accelerates Flash, it will take almost twice as long to get the same amount done as with, and you will lose input control if you button-mash the one spell you'll want to button mash. But don't worry, you'll only notice it at critical moments, and only cause you to lose your "perfect" completion on a couple 45 minute boss battles... before you learn. Or your RSI flares up.