I can understand why you would want 4 farms, Mines, and the like but 4 Armories? The game will not allow you to train any troops in another armory when one is active. So why was I instructed to build them and I have been up gradeing them all.
You were asked to do things as in introduction to the game, there was never any compulsion to upgrade everything equally.
If you wish, there is nothing to stop you demolishing those you no longer want, or need (you have to maintain at least one),
Though doing so may have noticeable effects on training troops.
You will find at some stages of the game there are advantages in having more of certain buidings.
Your idea that each barracks trains troops separately is mistaken. When you click on "troops" and select the troop type to be trained, the maximum number of troops that you can train is the agregate number for all the barracks that you have. If you have four you will be able to train more troops at a time than with one.
The general practice among well developed towns (500 billion influence plus) is to have two barracks. I have my doubts about this and suspect that while developing your town and army a somewhat higher number may be useful. As far as I know no one has done any testing (with different combinations of barracks and manors) or, if they have, it is information clans keep to themselves.
