Where can you walk through the wall?
There's this hippo that keeps moving around everytime you talk to him and then find certain treasures he's looking for before he does (He never gets there first) It's a room where you come in from the right and you can go up two small platforms and jump down to the left along a solid wall.
There's these bird head things, one coming from the left and one from the right trying to knock you away and there's a single fire where you go up those platforms.
Basically, that wall you can fall down, isn't solid, and you can open it up by jumping down and hitting it, then climbing back up and jumping and hitting it again. Once it's open, you can pass through, jump a gap and go through the next few screens of walls.