Establishments are structures in a territory that define its stats. These are preexisting mechanics built into the territories (meaning you cant build them where you please). However they can be upgraded. Upgrading an establishment cost supply and/or power tokens and takes up an action. Once an establishment is upgraded it cannot be changed to another upgrade later on. (farms have no upgrades).
The stats are as followed:
Muster - determines the amount of troops that can be mustered
Supply - determines amount of supplies gained
Defense - bonus to combat when defending the territory
Upkeep Reduction - reduces the amount of upkeep needed to pay at the end of the Year.
and Power - determines amount of power tokens gained and adds to your position on the World Supremacy track (this stat is not adherent to any kind of establishment).
There are Four base types of establishments:
Farm
Supply: 3
City
Muster: 1
Supply: 1
Defense: 0
Upkeep reduction: 1
Stronghold
Muster: 2
Defense: 1
Port
Muster: 1
Supply: 1