Lots of effort!
It looks like there was a lot of time and effort put into this game and it really paid off. It does what few other games on this site do... actually present a challenge.
Also, the graphics were amazing for a flash game. I completely disagree with the dude earlier that complained about the graphics being too good. That's the most insanely dumb thing I have ever heard. Graphics may not make gameplay. but it sure does make games easier on the eyes. Good work on that.
The controls were spot on. I see people complain about not being able to aim and shoot. Well no shit! It's not a shoot game, it's a strategy game. The hero is like a turret you can move around and should be used as such. The fact that you can't aim where the hero shoots actually makes it easier in my opinion. You place the hero where he/she is most effective so you can carefully exam where you want to put your next towers.
Bottom line is. the author spent a lot of time thinking about things like these and I think he pulled it off pretty damn well. That deserves a 5/5 and a 10/10 from me.
Keep up the good work!