Pokémon Red and Blue/Pokémon Mansion

'Buried somewhere in the rubble of Pokémon Mansion is the Secret Key that opens the Cinnabar Island Gym. There are plenty of salvageable items amongst the wreckage, as well as clues regarding the dangerous experimental Pokémon that escaped. Hunt carefully, as there are lots of powerful Pokémon here as well!'

The Cinnabar Mansion
The door to the Gym is locked, and rumors suggest the key is lost in the abandoned mansion. You can get through the mansion fairly quickly, but the random Pokémon that lurk here, especially a wild, can be fierce.

To get the key, take the stairs to the main part of Floor 3 (A to C), search the statue for a hidden switch (every statue here has one), then jump down the first balcony near a scientist marked point D. The stairs here will take you to the basement where the Secret Key lies. Flip all of the switches you pass and you'll find it without much trouble.

There are a number of valuable items and Pokémon here. Yellow players can pick up a and a. Red and Blue can get a, and Blue can also get an ultra-rare, Blue-exclusive.

The items include a number of stat-enhancing supplements, and two TMs. TM 22 is the most powerful Grass attack, Solarbeam, and TM 14 is the super-powerful Ice attack Blizzard, which you'll definitely need for the Elite Four. There is also a hidden Moon Stone in one of the pillars on Floor 1, right after you enter the mansion. There is a hidden Max Revive on Floor 3, at the top of the narrow path above marker A. There is a Rare Candy in the basement, in the room with the Secret Key near the top-left corner (above the plants). Also fight all the Burglars that you can — they'll give you good EXP and a ton of cash.