The Pokémon Mansion is a decrepit, burned-down mansion on Cinnabar Island. It got its name because a famous Pokémon Researcher once lived there.
To access the Cinnabar Island Gym, the player has to enter the mansion and find the Secret Key to unlock the door of the Gym.
The Pokémon Mansion has four different floors. Doors can be unlocked in the Mansion by pressing switches hidden in Mewtwo statues.
Amongst the rubble and wreckage is information written by the scientist who obtained a Mew and cloned it to create Mewtwo.
Finding the Secret Key
In order to reach the Secret Key, you must Activate a Statue on each floor, then navigate to the Jumping point shown below, and fall down to the basement.
Then, you’ll need to activate 1 more statue, and you’ll find the Secret Key.