I find mini-defensive towers are quite... boring. Why? because the dinamic of isengard base is way more dinamic.
I propose to upgrade those towers to the following idea:
-Once the wizard tower is purchased, we could convert that tower into a fire spiting machine, as the one we saw at 3.8.1
http://www.moddb.com/mods/edain-mod/images/isengard-fire-vent#imagebox[...] As the Fires of Isengard were a defense mechanism against the Ents mentioned in the books, we felt they were a much better fortress expansion than the mine throwers.
-Once purchased the wizard tower, we could upgrade that wooden-rock tower into a black-rock tower (as the expansion of isengard fortress -
http://media.moddb.com/images/mods/1/11/10679/sshot0003_1356540419.jpg ), but instead of firing arrows it will cast voice spells of Confusion or Words of deceive, etc. Why? read the following:
[...] According to some background material, Saruman had given his Gate special properties. Whether by some mechanism or spell, he was able to project his famous voice through the Gate itself, and so communicate with those outside his defences without putting himself at risk.