Protector

http://www.kongregate.com/games/undefined/protector

Yet another Tower Defence game. But if it has to be the same old game again then i want mine beefed up and pimped out.
This one features quite nice graphics, a kinda non linear campaign. Your mages (instead of turrets) gain XP and you can choose between two paths (specialist and  generalist).
With every succesfully finished level you earn a skillpoint which can be used to unlock advanced units and other stuff.
And it gets quite challenging, which is a good thing as i tend to to go back to work after failing,  which is the last good thing, you can always continue at the point you left.