updated
To access Team Rocket's secret hideout, examine the poster in the Game Corner. To do that, you'll first have to beat down one of their grunts:
Team Rocket Grunt
- Raticate (20)
- Zubat (20)
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Once he goes down, search the poster. You will find a secret switch and a way in. The place actually looks more complicated than it really is. The only thing that may throw you off are the conveyor belt tiles that automatically move you. Just experiment with different paths and you should be able to find plenty of items and get through the whole stage.
You need a key to get to the boss, so do that first. To get it, follow the stairs down to the northwestern corner of the fourth basem*nt level. Beat the trainer there to get the Lift Key. With the key in hand, return to the top floor and take the elevator down to B4.
Items
There are a ton of items in this stage, including the mysterious Silph Scope you need for the Pokémon Tower. Apart from the Lift Key, here's what you can find:
- Escape Rope
- X Speed
- Blackglasses
- Max Ether
- TM12 (Taunt)
- Moon Stone
- Super Potion
- Hyper Potion
- TM21 (Frustration)
- Rare Candy
- TM49 (Snatch)
- Calcium
- Silph Scope
Trainers
- Team Rocket Grunt
- Drowzee (21)
- Machop (21)
- Team Rocket Grunt
- Raticate (21)
- Raticate (21)
- Team Rocket Grunt
- Zubat (17)
- Zubat (17)
- Koffing (17)
- Grimer (17)
- Raticate (17)
- Team Rocket Grunt
- Machop (21)
- Machop (21)
- Team Rocket Grunt
- Rattata (20)
- Raticate (20)
- Drowzee (20)
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- Team Rocket Grunt
- Koffing (21)
- Zubat (21)
- Team Rocket Grunt
- Grimer (22)
- Koffing (22)
- Team Rocket Grunt
- Raticate (19)
- Raticate (19)
- Rattata (19)
- Rattata (19)
- Team Rocket Grunt
- Grimer (20)
- Koffing (20)
- Koffing (20)
- Team Rocket Grunt
- Sandshrew (23)
- Ekans (23)
- Sandslash (23)
- Team Rocket Grunt
- Ekans (23)
- Sandshrew (23)
- Arbok (23)
- BossGiovanni
- Onix (25)
- Rhyhorn (24)
- Kangaskhan (29)
Once you defeat the Boss, you will have the Silph Scope, so head east to Lavender Town and enter the Pokémon Tower. You can now take Route 7 and go through Saffron City to get there more quickly. Simply hand over the tea you got in Celadon City's Mansion to the annoying guard blocking your way. Voila! Now all guard houses are open.