A fungame, very addictive. Base tower upgrades are good, although the split shot seems to be a little too strong for the current cost.
The only other bad point I can say is that there does not seem to be a need to buy the recruits not on the walls(soldiers/knights), they can die and cost more to replace while base tower upgrades archers/canoneers are much more long lasting. Towers set to the sides are also a somewhat greater investment, as they are out of the way but have a large area they can affect during the whole game.
Maybe a balancing of costs/power might improve the choices you have set-up.
But in the end, great game and I have played through all three difficulties, which means you kept me coming back!