The Secret Door consists of two large stone doors surrounded by a thick stone frame and is sealed with five padlocks across the doors' widths.
The Secret Doors appear in every stage in Shadow the Hedgehog. Behind their entrance they have contents that can provide the player with extra help to complete a Stage. What they hold however, differs between stages. Some have a hidden route behind them with a flying vehicle that can be used for a shortcut, some hold ground-based vehicles and weapons to help fight enemies, and others contain Warp Holes.
In gameplay, the Secret Door cannot be opened until the player collects all five Secret Keys of that stage. When approaching a Secret Door, a number of locks on the door equal to the number of Secret Keys the player has collected will automatically break off; the doors will only open when all five locks break off. Upon successfully opening the door, it will be automatically saved into the game data, thus no longer requiring to open it again next time.