A Secret Passage!
The Kobold Hall was the estate of a minor lord who came to the Nentir Vale to establish his own demesnes. Ruined during the Bloodspear War, the old castle has been abandoned for almost a century. Kobold tribes from the Cloak Wood now lurk in its depths.
The party has been recruited by the dwarf Theldorthan to retrieve a cured green dragon hide, which he suspects can be found here, since kobolds raided the trade caravan that carried the hide.