Why choose Unity platform? In past year or more since i've seen and tried Unity it has been nothing but hassle. Install this version, but uninstall that one, restart Computer,... and it still not working, When it actually starts it behavious poorly on key commands. Here, i didn't get any automatic download screen even it says i should - yes, i could download it separatelly but i wont since Unity has terrible user experience. Sorry to say this but i wont be testing your game just because of the platform. A sidequestion: isn't there some other platform that installs, runs and behaves smoother?
I believe the idea about not being able to click the stacked items on the ground is you can't horde all the powerups for the boss - proper selection of towers and upgrades is the key to winning
sry for that, there can be overloads.