There is no key. It is opened by a quest; you need to talk to the seekers in the second floor of the barracks if you got them kicked out of the Black Boar tavern at the start of the game, but told Louis' it wasn't a big deal. If you tell Louis they were getting out of hand, they will be sent out to take care of a troll; after you get access to the full Broken Valley you can encounter them fighting and if you are quick can talk to Merrill to start the quest that opens the hatch.