The outer circle of your Base is where you can build Add-Ons. They attach to and extend the functionality of your districts with a unique ability and bonuses to specific stats. As you build Add-Ons you will see how your Base grows with each one. An Add-On can be constructed only on tiles that physically touch the district they belong to, so space will be limited and you have to chose which one to build and which to skip depending on your command style. Don't worry though, they can be removed and replaced so nothing is too permanent.