To get anything from here, you must be playing in Spring or Summer . If you're impatient, feel free to skip ahead to a Spring or Summer month, then re-enter the sewers. From the entrance, head west, either by Surfing in the nearby water or by walking along the platform along the north.
Once you reach the next bridge going down, head down then stop before the gap in the fence to your left. If you didn't grab the X Sp. Def in Autumn or Winter, you can grab it by now by Surfing into the water from the gap, then heading south and west.
When you've got it or otherwise and continue down from the gap, while on dry land, past the Janitor. Now Surf into the water to the south, then head west and hop onto the nearby land to your south. Use your Dowsing MCHN to locate the hidden Max Repel .
Then head west and Surf across the small gap. Nearby is a Black Sludge that you can... pick up. Surf to your right again, but this down follow the water south. When you reach the blockade, jump back to land on your left, then head west. Near the end, use the Dowsing MCHN to recover the Poison Gem .
From here, Surf into the water below and follow it east. You should spot an item nearby, separated from you by a fence. When the fence ends, go around to grab the item and discover it's TM41 - Torment . Head back to the water and Surf towards the land in the south-eastern corner.
There's a Scientist walking back and forth; let's surprise him!
Trainer Battle: Scientist
Ho-hum, that wasn't very exciting... Return to the water and follow it all the way west then when you reach the end, head north until you land on a wooden bridge. Surf into the water above and quickly hop onto the land to the north-west. Keep going north until you're on top of anothr wooden bridge.
Then face left and Surf into the water. Swim west, then north and land just before the blockade ahead, where there's a gap in the fences to your left. From here, head south a bit and use your Dowsing MCHN to find a hidden Ultra Ball . Then make your way north.
As you reach the corner, turn right. If you enter the door ahead, you can obtain a recovery item from the Scientist. Back outside again, continue right. Very soon there will be a T-intersection. For now, head right where a Janitor is scurrying around. Bump into him to trigger a battle!
Trainer Battle: Janitor
After cleaning up, return to the intersection and head north up the stairs.
Castelia City - Slums
Coolio, you're outside! Immediately head to your left to pick up the Repel just sitting around. Then cautiously head north. When you get close to the gang at the end, they'll challenge you to a brawl... Pokemon style!
Trainer Battle: Roughneck
Trainer Battle: Dancer
Trainer Battle: Guitarist
The gang will be so impressed with your skills that they elect you as their new boss. Sweet. You can now add "Castelia Gang Boss" to your resume. Now that you're free to go, be sure to grab the BlackGlasses nearby. Then go wherever you need to go!