If you find yourself on a server where Horde or Alliance stomp the opposite faction via capping all the Workshops, then this little trick will help you get your weekly Siege quest done when you otherwise couldnt.
Go to one of YOUR Workshops and duel someone nearby, Have them build a siege engine during the duel and it will be hostile to you since they created it, Kill it and get one of your 3 siege kills, rinse and repeat 3 times to finish it off.