what sewer entrance?... now, if you go to colossia you'll find more interesting things: by interacting with the crystal there (right beside the sewer entrance) you can go back to the first day of the game, choose to face Crayden before he can execute his plan and basically get the third game over off the game with a brand new scene with Laquadia. (tho this is merely to get the scene, after that you'll get a game over and be send back to before you can face crayden so you can return to the normal timeline)
is this before overworld map change?