The City Hall is also used to upgrade 4 of the buildings within the town:
The City Hall itself, which increases the amount of crystal and silver collected with taxation.
The Seaport, which increases both the number of areas available during expedition and the amount collected from expeditions.
The Alchemy Lab increases both the quality of the cards that can be combined and the rate of success of the combination process. It also increases the number of fusion slots available to select from.
The Blacksmith increases the enchanting and combining success rates.