In Mario & Luigi: Bowser's Inside Story, there are quite a few unused rooms without anything in them, most of them used for testing. One of the rooms appears to be a pipe in Dimble Woods leading into bowser's body that can only be accessed through the use of hacking. It was probably removed because once you've found all the attack pieces there you had no need to go back. Another one of those rooms are in Dimble Woods, along with what appears to be the Tower Of Yikk on the ground. This might suggest that originally bowser was supposed to fight it in dimble woods, but it was replaced with Bowser's Castle.