Nice but...
This is very good game, I must say. Many things are done great.
-graphics : great pixart. Really. It fits here nicely, too.
-gameplay : towers are varied, and nicely balanced. Game is really fun to play.
-interface : hotkeys are easy to learn, and controls are friendly.
It would *normally* deserve 10/10 for sure. Yet...
...It has issues. Serious ones. Like:
-interface : minor glitches, occuring rarely
-gameplay : ...difficulty curve. Only really difficult waves are usually first then, you can survive later ones with nearly no attention.
That is, game after some time gets *boring*. Really.
Also, if you manage to learn some proper tactics, you are able to get *enormous* amounts of bones, allowing you to raze nearly every single wave of attackers with only your spells, and this isn't as costly as it may seem. (I *never* finished a game with less than 1500 stacked bones in my "account").
Simply, humans are too weak, the only ones being actual threat - kamikaze, mages and warriors - are just too easily obliterated by two level 3 plants and some spells of yours.
I'd like to note that I'm not great fan of Tower Defense games, so I'm not some pro-uber-master of these, yet I easily won even Extreme level plays.
I'd give the game 5 and 10, but because of aforementioned flaws - 4 and 8.
(game is still really cool and enjoyable, after all)