As Luigi, go to Princess's Secret Slide and slide down until you get out of the walled area and you see the whole map. Now right before the hill, jump and ground-pound. You should stand up. Take a look towards where you just came; you should be able to see through the walls. If you manage to get on top of the area you just came through (try a triple jump), you can see that the other side is the same way.
Submitted by: Thor