I replayed the game with a sorcerer. Got Nalia's quest, cleared the keep, forged the flail. I didn't kill the nasty big treasure protectors because they didn't pay attention to me taking the 3rd flail head away.
I left the keep to get some better weaponry. Big mistake. When I came back someone from the Roenall Clan greeted me und denied access via the drawwer bridge. I left him to his ramblings and headed for Nalia's secret entry. Nice surprise, it was gone.
Did I miss something? Will I be able to enter the keep later in the game?
An attempt to gain access by killing the Roenall guy was unsuccessful.

If you killed Torgal (the boss troll) and Nalia went through her little speech then the quest is indeed over and you cannot reenter the keep.

I am pretty sure I have read posts of those who have fighters that the room with the golems no longer has the golems or the treasure after you finish the quest anyways. Not 100% on this since I have never experienced it myself though.

The golems do disappear after the quest is over, so it's best to see if you can kill them before that. If you get the chance.

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Lord of the RaBid Fruit Trees. ChAos rules all! Bwahahahaha...
:1drgat1:

Don't bother about Nalia's castle after killing the boss. Unless your main character is a fighter, there's nothing special after completing her quest.