 FACT: When the gate to Mordor is opened, it is clearly seen to be as  thick as the wall, and each corner is seen to be a right angle.  There is no space at all in between the gate and the wall when it is  closed, later on in the same scene. However, the gate opens on a  curve, and so would get stuck every time it opened or closed, unless  the wall was somehow retracted. We do not see the wall retracted at  all, thus it is physically impossible for such a gate to exist.