Headquarters is the starting building for all players, as well as the most important building.
It has 3 purposes:
Provides the starting 10 population
Act as the starting military building with same capacity as a fortress, but with less influence range same as barracks
Act as a warehouse to supply materials for construction
If your headquarter falls to enemy, your tribe will be wiped out from the game. Vice versa for enemy headquarters.
Once Administration has been researched, Headquarters will have a different appearance that resembles a fortress, but with the center tower replaced by warehouse.