The gym is located in the south of the city, just above the harbor. Its gym leader is Koji in Brown and Prism, and Ginta in Rijon Adventures, and both use Fighting-type pokémon. The Fist Badge is awarded to trainers who defeat him, allowing the player to use Surf outside of battle.
This harbor is the final stop of S.S Anne, but when the player gets the S.S. Ticket, the ship isn't in there. However, the ticket is not necessary to get through, since the player can bypass the sailor by surfing in the water and landing below him.
In the harbor, a truck is parked on the right. A CoronetStone is hidden inside (which causes Magneton to evolve into Magnezone). This is a reference to the rumor about a Pokéball containing Mew under the truck in Pokémon Red/Blue.
Other points of interest
In the small house in the northwest of the city, a fisherman gives the player the Old Rod.
In the house to the left of the Pokémon Center, a girl wants to trade her Chansey for the player's Pidgey.
Pokémon Prism 
Pokémon Rijon Adventures
In the house to the bottom left corner of town, an old man offers to trade his Jynx for a Poliwhirl.
Castro Valley's layout is very similar to Olivine City's prototype design.