Marching up and down Britain with occasional breaks for bloody and gruelling battles burns a lot of calories, so armies need to be well-fed. At first, you'll be limited to a single army thanks to low food, but you'll have enough money to build and upgrade some farms, allowing you to expand your forces in a few turns. Resource buildings like farms and mines are no longer constructed inside settlements and are instead represented by small villages that don't have any garrisons. This makes them a lot more vulnerable to attack, and it also means you don't have to besiege a settlement if you only want more farmland. Once you've upgraded your own farms, then you start looking for ones to pinch.