I had never heard of these and had to look them up:

"Judas gate": A small door or gate (still large enough to pass through, I think) set into a larger door or gate (as in a factory).

"Judas door": A small hole in a door through which a person can spy without being seen from the other side. Or sometimes also the same as the Judas gate.

"Judas hole": A peephole