Farmland, Lumber Mill, Quarry, Iron Mine, and EXP Crystal Mine all produce resources that are essential for city improvement, such as Food, wood, stone, steel, and EXP crystals.

You can construct these resource buildings in small plots, and you can construct multiples of each type.

 The higher the level of your resource buildings, the more resources you will receive.

After the castle reaches level 10, you can construct the quarry, and after the castle reaches level 15, you can construct the Iron Mine.