When you take over a keep why don't they add in a vendor that will repair your siege equipment similar to how we pay to fix our gear. It could costs gold or AP. My reasoning is this. If you buy some of the AP exclusive weapons and are very active in siege you wind up spending more AP then you make from attacking and repairing walls/doors. It would be nice to be able to repair those items with gold or a lesser value AP. It would def lessen the blow for those who want to siege and repair keeps.