Through unforeseen consequences involving a map that was cursed and thereby improperly drawn decades ago, it turns out that Uther's kingdom was not as large as he believed it was. Following his death, Arthur is drawn into a land war after he can't prove the castle is on the correct side of a border. Merlin, Arthur, Morgana and Gwen are forced to defend the castle from trolls, at one point shouting they weren't destined to live under a bridge. Merlin magically convinces the trolls to sell it back to them, so they can stay there.

(I stopped watching Merlin so long before the series ended, but hey--I did try with this summary)