Are you jesting???
I am sure the makers of this sim made all those upgrades just for fun, all the work considered that has to go into additional towers etc...."<Ironie",... tsss
ITs usually a goood idea to approach such things with LOGic..,if you do, you will realize that the effort for each of those features is so enormeous, that no one would put them in just for fun without any function.
Bugs and unfinished products excluded.