Farm, Lumber Yard, Quarry, and Iron Mine are the buildings where you can produce Food, Lumber, Stone, and Iron. 

Each building will be indicated with an icon on top when the production is complete.

Upgrading these resource buildings will increase the hourly production and the amount of resource it can accumulate for collection.

Quarry is unlocked when you reach Castle Lv. 10, and Iron Mine is unlocked at Castle Lv. 15.

Resources not collected will not be protected from enemy's attack. So collect your resources as often as possible.