Fields provide they owners with resources every hour. The different types of fields will yield wood, stone, food or iron. Every new field has automatically level 1, but it can be expanded up to level 20. Every level is more expensive than the other and the last level requires an expense of 10 salt crystals. Depending on the type of field, there can be differences in the production. When build on ordinary grass, one will end up with a simple field:
Beside the simple fields, one can also use natural fields, like forests or mountains, lakes and paddocks to extract more resources. These fields yield three times as much resources as compared to the simple fields. That's why it is probably wise to develop these natural resources first, before building the ordinary fields.