The Door Factory is the final level of the Monsters, Inc. world.
In the first part, you have to navigate your way to the tel port to the next part.
In the second level, you have to get through the doors to the tel port, including destroying red doors with bombs.
In the final part, you have to get to the Door Stack and lower it down, getting rid of Bad Doors as you go. Once you've lowered the platform completely, you can then rescue the trapped guest.