They could build everything there.
In my "vision" for what CZs could be, I would say player ships couldn't really do much to capitals at all. The idea would be escorting subcapital ships on your side to strategic targets so they can put the hurt on, while trying to destroy subcapitals from the enemy side. All of this with interesting backdrops like shipyards, refineries, asteroid mining complexes (I mean big facilities, not just a pile of rocks with some spaceships shooting lasers at them) with lots of stuff going on around you would make for really awesome and interesting firefights.