Gene Roddenberry stated that the second door added to the bridge in The Animated Series was a "second turbolift", in case the first one gets stuck. But Franz Joseph postulated the more logical theory (IMO) that it goes to an 'outer ring' corridor of the bridge, which in turn
has access to an emergency exit stairway down to Deck 2 and beyond, as well as the unseen toilet facilities, access to external airlock, etc.
I can't recall immediately, but I do seem the vaguely remember that the only time we ever saw anybody use the door was once when Kirk entered
the bridge from it. I think it was in "One Of Our Planets Is Missing", but I can't remember clearly if that is true or not.
there any evidence on screen as to where this door goes, exactly? If so, are we meant to assume that it was there in TOS, even though it's self-evidently not
on the actual sets? Or, like the additional work-stations on the bridge of the D in Generations, are we meant to assume that in-universe, it was a structural addition made during some refit between the original series and TAS?