The safe is a specific smart contract that hold most value of the game. When a user buy a game's item, that send most of the value to the safe and make it available to win.
The safe is located in "The Dead's hand" Casino and is build to hold most of the value of the MetaCity infrastructure. It was built to handle the Casino's property at the begining but was so strong that it was used to hold all the value of the city as a bank should do.