Find a Bilgewater Blasting Cap on a Bilgewater Sapper, and then plant it on a Domination Point Demolisher.
The Horde got the bright idea to send Bilgewater goblins to attack our quarry. They've been packing heavy explosives - heavy explosives that will react with the ammunition in their demolishers. Kill the Bilgewater goblins until you find an explosive, plant it on one of the demolishers... and BOOM! Siege ended.