Luigi's Mansion/The Hidden Mansion

After you complete the game, you will return to Professor E. Gadd's laboratory. When he asks you where you want to go, you'll notice a new option, the "Hidden Mansion". This is essentially the game's "hard mode". In it, the entire Mansion is mirrored. So, for example, in the Foyer, the Toad on the left is now on the right. Also, you must play this Mansion in the Sidestep control mode. Ghosts will lose health quicker, but put up more of a fight to escape the pull of the Poltergust. Because of all this, there is much more treasure to be found in the Hidden Mansion, making it more rewarding, and easier to achieve a Rank A mansion at the end. (In the PAL version, this is the only way to achieve Rank A).