Economy
Economy-related buildings are highly necessary for any city that desires growth. These buildings all provide you with the essential resources needed in the game (Food and Gold primarily).
All of these buildings increase production with every upgrade so it is best to upgrade these buildings regularly. Just the same, it is important to know what to focus on when upgrading--you will have to choose if you would like to increase your resources or perhaps focus on building and strengthening your Army. To upgrade these buildings, you would have to spend Food or Gold.
Most of these buildings are available as soon as you reach the Iron Age. As you advance through the Ages, the allowable number of buildings increase as well.