How do you get to the path behind the Pokemon league castle in Pokemon Diamond?

Answer 1

There is no path behind the Pokemon League Castle. If you heard that from a rumor, then don't listen to them. Not all rumors are true, you know. But if you did want to get behind the castle, you'd have to initiate the Walk-Through-Walls action replay cheat so you can go through the building and reach the back, but I don't have the code, so I can't help you there.

There is a path to the right of the Pokemon League Castle, which leads to the place you can capture the legendary Shaymin, but it can only be reached by means of a door in Victory Road after obtaining the National Pokedex or the Walk-Through-Walls cheat.

you actually don't need the walk through walls to ge to it. in victory road, instead of going down towards sunnyshore, try going to the right. Eventually you should find a place with some fog, use defog and continue to the right, not up. Eventually u should end up at a doorway with light coming out of it. That is the last door, and then you're at the end.

You can get to the path, so don't listen to the things above. But first you must achieve the National Pokedex by seeing all the Pokemon in the Sinnoh Pokedex and then talking to Rowan. Once you have done this, fly to the Pokemon league and go down the wall with rock climb. on the right you should see a cave hole kind of thing that was previously blocked. find your way there and go through- you will meet Marley and choose to go with her (she will later be available to double battle with at the Battle Tower), once you have found your way to the end she will say goodbye and leave through a cave hole on the right. Before exiting through the cave hole go up the stairs on the left by the water, and pick up the Big Pearl. Exit, walk up the long stairs and leave through the opening with light beaming through it. You are now in the 'Mini Sinnoh' behind the Pokemon League that was impossible to reach before.