The Headquarters is the main building in the Alliance City, similar to a Stronghold. You must upgrade the headquarters before you are able to upgrade your other buildings. The Headquarters gives you an overview of the Alliance City. Upgrading this building increases the maximum resource donation per march:
Donation Size Bonus
To Build Level 1 Alliance City:
The following resources need to be donated to the Alliance City in order for construction to begin. The R5 of the alliance will find the Alliance City on the Alliance screen where they can begin begin construction once sufficient resources have been donated.