Sieges are at the very core of the Mount & Blade experience. They are brutal affairs, the outcome of which can change the entire course of a war. And in Bannerlord, where villages and their respective resources are tied to castles, this is especially true, as the loss of a castle could result in the loss of a vital resource which is needed to feed the war machine.
Read more at: http://www.taleworlds.com/en/Games/Bannerlord/Blog/105