This one is simple enough, and it will inevitably come as you play through your campaign. You may find that you’re not able to besiege castles or towns initially. This is because you need to declare war with the specific faction first. You can do this by attacking an army from a specific faction, or by joining another faction in a fight against them. You will then be able to select any of your new enemies’ castles or towns, and choose to besiege them. you’ll have the option to select specific siege weapons to make things faster, or you can choose to starve the enemy out if you’re unable to construct any siege weapons.