You know... I just realized that Murphy's Law works perfectly here... If anything wrong can happen, It will happen, in the form of the piece-you-needed-least-which-causes-the-tower-to-topple-over.
Upgrade information put is also rather confusing, It says I'm going to have 150 range on a tower if I upgrade and it only goes to 130. And really? All the towers have the same upgrade price?
You know, sometimes I question the competence of these guards. There was this one that happened to have the dead target pretty close by and he never turned around to look.