I agree with AlexAUT in his suggestions except for the second item. I don't think it makes sense to occlude towers with towers. My reasoning for this is that the board is fixed most of the time, as in, unless you're placing new stuff, the same turrets won't fire at the exact same points by occlusion.
This is really undesirable because it will reduce the damage dealt by a constant amount every wave. If this is what you want, just reduce the base damage of turrets and achieve the same. This would only make sense, in my opinion, where placing turrets was a crucial part of the strategy and you couldn't just swarm all the borders in turrets, like you usually can in the genre.
Anyway, keep it going, good job so far
![Cheesy :D](https://www.sfml-dev.org/forums/Smileys/default/cheesy.gif)