Keep in mind, that they are not like ordinary castle sieges. At clan hall sieges, you have to kill a boss (An easy one... Low stats, no Petrification, no Raid Curse...) in order to conquer it. The clan that kills it first, takes over the CH, and the siege ends. Besides, when a siege begins, the clan hall goes back to the NPCs, no matter if a clan possesses (lol strange word. First time I write it in this form :D v. "to possess") it. Therefore, siegeable clan halls, cannot be defended, in order to be retained by the clan that owned them before the siege. A clan would need to re-attack the clan hall and kill the boss first, in order to keep it.
Well, that's some of the basic stuff you need to know. I'm not really in expert at siege CH, so you'd better consult guides on the web.
P.S. No idea xD