7th Stand User Wiki

Hidden in every city throughout the game (barring the foggy town where you fight Justice) is the entrance to the Ghost House, or Emporio's room. Sometimes, when you face a wall that appears to have nothing there, you'll hear a knocking sound. Knock a couple more times to be let in. These cannot be accessed while playing as Josuke. There are a couple things of note in here:

If you try to touch the computer on your first playthrough, Emporio will ask you not to do so, but on a New Game +, you can use it to access the Stand Summoning Program. If you talk to Emporio after saving a code in the first slot, he'll give you his own, allowing you to battle him.

By inspecting the sink, you can receive an infinite amount of Bottled Water if you're patient enough. The door on the left leads to the Sanctuary, where you can speak to the ghosts of enemies you've defeated and fight a few secret bosses once your level is high enough. The door on the right won't lead anywhere for most of the game, but checking it after locating Dio's mansion and returning to Cairo will take you to the Rolling Stone Maze.

A list of entrances to this room in each city is below:

Japan The large brick building south of the cafe [1]
Hong Kong The alleyway to the left of the 7-Eleven
Harbor The shady part of the wall next to the hotel
Singapore Alleyway in the building to the right of the Kopi shop
Calcutta East side of town, 2nd building up after crossing the bridge
SPW Foundation Camp Wall to the left of the tree in front of the inn, near to the man who complains about odd noise [2]
Sundarbans Building below the radio tower, on the left side of the wall
Varanasi To the left of the lake in front of the souvenir stand, on the wall of the building with shutters, four tiles to the right of the man who mentions RC Helicopters
Karachi The building in front of the mango stand with zombies around it, on the right side of the wall
Fishing Village Building with a telephone pole next to it, in front of the inn
Aswan Northern part of the map, windowless brick building by the river
Kom Ombo Building on southmost part of the map with a telephone pole next to it, in front of the bar
Edfu Hotel building, in between the two windows to the left of the entrance
Cairo Very end of the hidden alleyway accessible from the side of the hotel [3]