Note: As this is the hub level, there are multiple floors, leading to other locations.
Upon entering from outside, the main lobby is a big room with 3 stairs: one in the middle and the other two on each side. To the left is Bob-omb Battlefield, which is a large room with a Bob-omb Painting. Closer to the middle is a door that leads to a room with three pictures of two snowmen, the middle leads to Cool, Cool Mountain. The closest door from the left to the center leads to a long corridor behind the stairs and the stairs to the basement. On the farthest right is the Jolly Roger Bay room, which also has a secret passage. Closer is a room exactly like the Cool, Cool Mountain room, but with one painting of Whomp's Fortress. Closest to the right is another door which leads behind the stairs. Going up the stairs, the middle door leads to the upper floor. In the right door, there is a room with 3 paintings of Princess Peach, with the right one leading to another secret level. The door on the left leads to a long hallway with a hole at the end leading to Bowser in the Dark World.
If Mario enters the doors closest to the middle staircase in the lobby, he will be in a hallway with stairs and a hallway. If Mario goes down the stairs, he will enter a room that has the entrance to Bowser in the Fire Sea. There is also a slide blocked by a glass structure. Going down the steps in this room take Mario to a long hallway which then leads to the biggest part of the basement. Going left goes to a dead end with a Toad. Going right leads Mario to a partially filled water area. Here, the Entrance to Lethal Lava Land is in the middle of the area, also on a small step. Going left will lead Mario to a supposedply dead end, but jumping through the wall takes Mario to Shifting Sand Land. Finally, going right leads Mario to the room with Hazy Maze Cave and another room. The room with Hazy Maze Cave is a platform, with stairs on one side, and in the very middle is an inky, purple pool. The other room on the basement's East side will take Mario to a room with lots of water and two pillars. If Mario ground pounds on these two pillars, the water in the room will drain completely (this also drains the Moat Water). At the end of the water filled area, there is a door that once the water is drained leads Mario to the moat. To the players right at the side where Mario enters this room, there is a staircase leading to the earlier mentioned slide in the first room.
After Mario beats Bowser in the Fire Sea, he can go through the door at the top of the lobby staircase. Going through this door leads to a spiral staircase. There is also a glitch that can be performed here where of Mario jumps in a spot right before the staircase, he will clip through to the top of the staircase. After going through a door at the staircase top, Mario will be in a circular room with another staircase and many different paintings, as well as two separate rooms. The enterance paintings in this room are Tall, Tall Mountain and Wet-Dry World. If Mario enters the door that has nothing around it, it will lead him to a cross shaped room with a small, normal, and large sized painting of a young Goomba and a big Goomba. Jumping in either the small or large painting takes Mario to Tiny-Huge Island. The size of the level also depends on which of the paintings Mario enters. If Mario enters the door with a small alcove around it, it will lead him to a room with a Lakitu, a mirror, and Snowman's Land. If Mario has the Vanish Cap unlocked, then Mario can go through the glass and enter a door leading to a pure white room with a Star. If Mario goes up the staircase, it will lead him to a star door that requires 50 Power Stars. After this door, another circular room is met, with a large clock in the middle, two alcoves on the East and West end, and a double staircase leading to another star door which requires 70 Power Stars (80 in the DS version). Jumping in the clock leads Mario to Tick Tock Clock. Also, if Mario enters the clock at a certain time (3,6, 9, 12) the clocks mechanics will act different. If Mario enters when the moving hand is at 3, the clocks inside will be gravely slow. At 6, it is completely random. At 9, it is very fast. At 12, nothing in the clock moves at all.
If Mario enters the alcove on the right of the room, he will be sent to Rainbow Ride. If Mario enters the opposite alcove, he will be sent to Wing Mario Over the Rainbow, which is likely the hardest level of the game. Finally, if Mario enters the 70 Star door, he will be sent to another staircase. To note, Mario actually can enter the door without 70 stars (80 in the DS version), However, if he doesn't have the required stars, the staircase cannot be climbed and will only loop endlessly, hence the name "Endless Staircase". If Mario does have enough stars, the staircase music will not play and Mario will be taken to a small room with a hole. Jumping in the hole will lead Mario to the final Boss level, Bowser in the Sky.
Inside the Jolly Roger Bay room
- Interestingly, the internal order of the castle's floors is different than the final ingame order, with the Upper Floor being "Inside Castle Area 2" and the Basement being "Inside Castle Area 3" despite the player not being able to enter the Upper Floor's doorway without having beaten the basement's Bowser In The Fire Sea boss battle first.
- This hints at the developers having had a different order in mind back in an earlier time when they were still working on the game only to later rearrange the levels.