The Bastion (Bastion)
Since it's right there in the title you can safely assume that your hub zone will feature prominently in the story. Gameplay-wise it contains all the important buildings and facilities The Kid will need in his adventures, and you choose which ones you want to build first.
In the uniquely torn-apart post-apocalyptic world of Caelondia The Bastion serves as your only safe place, as well as your ultimate salvation.