Could I suggest an update to the Q/A on the first page? Specifically:
Q:Why can't i siege a enemy castle?
A:You have to be on bad terms with a faction before you can siege castles and take other hostile actions. Try doing something hostile like attacking that factions villagers or attacking a factions Lord. Make sure you installed the mod right and that you have the latest patch! All the contents should go in a folder under your module directory in one folder.
Castle should be Town.....you can lay siege to a castle w/o declaring war on the faction, doing so will be the declaration of war, but you cannot besiege a town w/o hostilities already being underway.
I haven't checked this myself, but reasons you might not be able to siege an "enemy" castle could be because they aren't the enemy. If you are a vassal or merc, perhaps your king made peace with the faction you were about to siege, but you didn't notice. You cannot start a war when you are a vassal or merc via hostile actions, only the king or the enemy faction's ruler can do that. You also might not be able to siege when still under the duration of a truce, non-agression pact, or any other treaty. Not sure about all of that though.