SPOILER
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If Nalia isn't in your party she may be in the stockade to the south of the keep when you have cleared out all the trolls if she is not in the entrance hall. Go and talk to her to finish the quest. Be warned that if you tell her you don't want her in your party she will disappear from the game. If you agree to take her then choose to drop her straight away she will still be available for you to use later.
If Nalia is in your party, she should speak to you when you walk out of the keep towards the drawbridge.
Either of the above should give you the option to take the keep as your stronghold as part of the dialogue.
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas and The Ladies Guild.