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Art by Suhaito

The Gods did not remain idle after the defeat of the Titans in the Second Titanomachy. Foreseeing a time where the Titans would rise again, Odin worked alongside the greatest runic Druids he could find - regardless of their nature - to construct the perfect Citadel, which no Titan could take down, in the even of a catastrophe. Some suspect Odin's paranoia at the inevitability of Ragnarok is at the heart of this idea, but it remains that the Impenetrable Citadel was seen favorably by those looking for a place to ensure the Titans could not get to them.
Since then, the Citadel has been under the guardianship of Heimdall, and many among the Gods keep their most treasured possessions in the great vaults hidden in the Impenetrable Citadel, safe in the artificial Otherworld's defenses.