Bowser's Castle

From SMWiki

Bowser's Castle is the final level and is located in the Valley of Bowser. It has the final boss battle in the game. In the original, it has two parts: the Front Door and the Back Door.

In the Front Door, Mario gets a choice between 4 rooms. Every room is a small challenge. After Mario finishes one of those four rooms, Mario gets a choice over another 4 rooms, which work like the first. After that, Mario finds himself in a dark room with Ninjis and Mechakoopas. In the Back Door, Mario skips the four-room system and starts in the dark room.

Finally, after the dark room, Mario finds Bowser, and the final fight begins.