All the buildings your territory has are upgradable. You start off at level 1, but with a little bit of time, patience and a small investment you will upgrade each and every one of them. Every upgrade will require some resources as well as some people to build them (make sense, right?). So you can not upgrade all your buildings at the same time unless you have some extra troops at hand. I suggest always keeping your buildings at the maximum level possible for your current level, because that is a great way to gather resources as well as keeping your CP strong.