Level 0 - "The Lobby" / The place where you end up when you noclip for the first time.
Level 1 - "Lurking Danger" / The second level.
Level 4 - "Abandoned Office" / The fifth level of The Backrooms and one of the safest.
Level 2 - "Pipe Dreams" / The third level of the Backrooms and one of the most dangerous ones.
Level 3 - "Maintenance Tunnels" / The fourth level of the Backrooms.