[Mirac Honorguard]

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I say, just hack him up in the smugglers cave (when you can ask him for the entrance).
There are 2 other ways to get into the castle, if you like action, just wander around in the town and get attacked by mercenaries and monks, one of the monks holds a key to the gate OR you can talk to the tavernkeeper, ask him about an entrance, he'll sent you over to the house on the north-east cliffs, there's a couple around from calimshan (however you spell that) they have the key that you can use in the graveyard (watch for traps!).