Headquarters has the biggest avatar in the city, excluding Walls. It is prebuilt with a new city. Higher levels unlocks more resource plots and allows you to conquer more Wilds.
The headquarters has three tabs: Overview, Buildings and Wilds.
Overview gives you the options to change taxes and change you troop's status (hiding or defending). Below the two options is the Resource Production Summary, which generally shows your Base Production, Production Bonuses, Upkeep (food only) and Current Production.
The Buildings tab lists the structures you have in your Field and City. Note that it shows upgrading buildings at their current level, not next level.The Wilds tab states the Wilds that you conquered, their coordinates, level and status. There is an 'abandon' option for each Wild.