It's actually sort of refreshing to see a tower defense game with some story attached to it, and the premise in itself is actually sorta cool, but it never really becomes anything but something that could have been cool.
Gameplay-wise, it was mediocre, nothing new, but fun enough I guess. I could've really done without the small segments after each level though, walking through darkness with melodramatic phrases everywhere is neither engaging or harrowing, it just gets tedious.
As for art, it was alright I guess. Lots of lazy gradients and unimpressive art, but that's quite commonplace.
The big problem with the game though is that apparently you either forgot to bugtest it, or just got too lazy to fix the bugs that were there. Either way, you really should have taken the time to work them out, because so many of them are incredibly annoying, and some downright gamebreaking. Non-working upgrades, turret not responding to you pressing the fire button, space button becoming unresponsive and so forth.