The Mansion is the place in which the Nations are trapped in. It is still unknown how the Oni, or Steve, came in there and what is the meaning of the Mansion in the first place.
The first floor leads to the annex, the basement, the 2nd Floor and leads to the outside. It contains 9 rooms, including a Bathroom, the library, 2 Tatami rooms, 3 secret rooms, and 3 hallways.
The second floor leads to the 1st floor, 3rd floor and the safe room (after it is unlocked). It contains 6 rooms, including 5 bedrooms, 1 dining hall, and 1 hallway.
The third floor leads to the 2nd floor and the 4th floor. It contains 2 rooms, including the Piano room, a room with the many shelves, and 1 hallway.
The fourth floor leads to the 3rd floor, 5th Floor (after it is unlocked) and a hole leads to the Piano room. It contains 2 rooms, including the livingroom-like-room, a room with the red chair and Mochi, and 1 Hallway.
The fifth floor leads only to the 4th floor. It contains 3 rooms, including the Clock-Moon-room, the Oni room, a number room which has the Key to the entrance, and 1 small hallway. It is discovered by the nations much later in the game, although Italy was already aware of it being there.
The Safe roomEdit
The Basement leads to the 1st Floor and the big hole leads to the outside. Without a ladder, which was destroyed by Steve, the Nations will not be able to climb up. Romano and Spain are later going to jump down to the trapped Nations and much later Belarus and Ukraine are communicating with Russia through it. The Basement contains 14 rooms, including the jail room, the cave, a few normal rooms and a few hallways.
The Annex leads to the 1st floor and is very late discovered. It contains 6 rooms. Any 'Steves' that appear in this area seem to be more powerful than other ones.