From SMWiki
After completing the Valley Fortress, the Back Door is accessible. This level starts Mario off in a room that has a light. To activate the light you press a red switch. There are several Mecha Koopas and Ninji in this area. In addition, this is the only part the game where you will encounter Ninji, an enemy that is strangly absent from the final credits. This room is right before Bowser so that he doesn't have to go through any of the eight doors from the Front Door. There is also a midway point right before the boss door so that if Mario dies during the fight with Bowser, he doesn't even have to complete the last room again.
