The Chancellor of an Alliance will be able to found a Headquarters for the Alliance, similar to founding a new City. Click on an empty Plain and you will be given the option to create your HQ. You will only be able to have 1 Headquarters active at a time and will be placed on a cooldown before you can create or close another HQ.
Alliance Headquarters are not always open to attack by enemy forces. If you find an enemy HQ on the map, you will be able to view the HQ’s information and send the Chancellor a message, but you will be unable to engage in combat outside of special combat events. Check the Caravans or forums for information on the current schedule or events.
You now have an HQ Messages tab for your in-game mail back in your personal Cities. Your Alliance will be notified when HQ buildings are upgraded, items are donated, the HQ is moved, and other similar actions. Check your messages regularly!