In order to conquer the enemy castles, you will need many knights. To keep their morale up, you will need gold, and for the weapons you will need iron and coal. These raw materials are found in mines. The mines must be provided with food and the raw materials must be melted in a foundry.
The food supply is ensured by the fishermen and the farmers, but they need tools, as do all other workers. These tools are made by smiths. To build huts, houses, castles you will need construction materials and workers...