idk where you are in the main quest, if you've already reached the docks speak with the people in the tavern, one of them will give you the key to open the cliffside dungeon.
If you haven't reached docks yet, do the quest for a merchant that lives in front of the inn at the center-west market zone.
(I assume you've at least talked with the hermit that lives outside the west gate.)
The whole guard/giantess sequence is just one large bad ending, there is no hope once you pick it.