I agree with others that the gameplay is simple and fluid enough that the art doesn't need to be overly fancy or complicated. However, there are 2 small "bugs" that should be addressed:
1) The range circle indicator around each tower should be representative of the tower's current range, which would increase as it is leveled up. I just found it to be kinda irking from an aesthetics standpoint when a tower wouldn't shoot an enemy until the enemy was almost next to it (instead of right away at its range's limit)
2) There needs to be a way to deselect placing a tower. This becomes especially important on level 8 as there is very limited space to place towers (also it would be nice to be able to place them in maybe the grass squares right alongside the paths, but that's a different topic). Sometimes I would accidentally press a hotkey for a tower (very good idea, btw) and then not be able to unselect the tower in order to cast a spell or upgrade a tower already placed. This was troubling on level 8 as I couldn't simply place the tower in an out of the way space and then sell it (I had no open places to put it).
One other side note: From a map usage factor, it seemed like lots of the map was wasted when the majority of the path was only 1 square wide (again referring to level 8 for example). This is where the possible grass spaces mentioned above could come into play, or maybe extending the paths to a 2 space minimum. I realize that this creates levels that are a lot trickier due to limited building space, but then the game becomes upgrade....wait for money....upgrade...wait for money...instead of "sell X tower and place Y tower there since the current enemy has armor against tower type X"
Just my thoughts on small improvements. Other than those things, though, great job on the game. It is very simple and fluid, yet not of a boring nature. I like having multiple paths and different spawn points at different times.
Hint to players: It's always beneficial to corral the enemies as close as possible to your towers for as long as possible....basically like a labyrint....wait...THAT'S why the description said "build labyrinth's of towers..." Ah ha!