Not a bad game by any means, though there are some balancing problems.
Agiles are too overpowered- the fact that they basically ignore two of your defenses is outrageous (and you dont get access to the ball enough to make up for it). Since you can't use spikes on them, it depends on luck to hit them with the crossbow- and you often dont have walls or balls to fight them.
The last level is pretty tough too- I stopped playing at it because i lost my patience with units constantly smashing through defenses (the neverending stream of badguys is rather unfair) despite my best efforts to keep them down. Eventually, a unit (yep, you guessed it, an agile) made it by my line of bows and got me.
Still, the concept is lovely and the gameplay isn't that bad at all :) kudos.