I thought I'd finished it.
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Its outer structure is simply a large stone skull and is where Captain Lefwee hides his treasure map. Bentley and Penelope infiltrated Skull Keep in order to retrieve the hidden treasure map that will lead the Cooper Gang to Dagger Island in search of Reme Lousteau's stolen diving gear. LeFwee also put Penelope as prisoner inside Skull Keep, although in actuality it was a bomb disguised as Penelope used to eliminate Sly Cooper; LeFwee hid the real Penelope inside the lower deck of the Death Hen (LeFwee's personal pirate ship). Once LeFwee was killed, the keep was abandoned.