Fort Hard Knox
From the Super Mario Wiki
Fort Hard Knox is the piggy bank-shaped treasury of Spy Land.
In The Super Mario Bros. Super Show! episode "On Her Majesty's Sewer Service", Koopfinger and his goons robbed Fort Hard Knox, and managed to make off with nine-hundred and ninety-nine billion trillion billion million and one Gold Coins.
Other than a gate, Fort Hard Knox has no visible security, which might explain why it was so easy for Koopfinger to plunder.