Just a thought, I think that just military star bases should be able to restrict enemy war ships from entering their zone of control.  I would like to see them force enemy ships to engage them and destroy/disable them before they could move on.  Yes they can add effects to help fleets in their area but making them a threat would help protect planets much better.  even help defend your borders a bit better.  Now if you have the resources to dump into a lot of military bases you can make a great defense, But it is going to cost you.   It might need to be a tech option added in the tech tree.