"It" is referring to the door. It is understood that the last noun (a person, place or thing) that was talked about in the sentence before is what you will continue to talk about until a new idea is introduced (when they change the subject). "....hugely huge metal door at the rear of the room. It.... " The last thing that was talked about was the door, not the room. "at the rear of the room" is only describing where the door it. A description of a location is not a noun, it's adjective (a word that describes a noun or verb). I hope this helps. good luck with your studies. Ask for help any time