BUT IF (the entrance to a blind alley) HAS THE SHAPE OF A DOORWAY THERE IS NO NEED TO REDUCE IT EVEN THOUGH IT IS WIDER THAN TEN CUBITS
What is not clear is if this “shape of a doorway” refers only to the width, or to the height as well. If the cross-beam is too high (over 20 cubits) but it still “has the shape of a doorway” does it still need to be reduced?
And what is a “shape of a doorway”?
On both these points there are many debates. Perhaps a doorway is defined by the ability to have a door (even if one is not present). This might be shown by marks for hinges. There is also a mention of “Semitic” doors (coming from the name of Noah’s son Shem – as in, those from the Philistine or Palestinian areas) which may be doors without side-posts or without lintels.
What about an archway?
R. Shesheth met Rabbah b. Samuel and asked him, ‘Has the Master learnt anything about the shape of a doorway?’ — ‘Yes’, the other replied, ‘we have learnt: An arched [doorway], said R. Meir, is subject to the obligation of a mezuzah but the Sages exempt it.
Since a doorway must be a minimum of 4 handbreadths wide to have a mezuzah attached, does an arch count? After all the top of the arch is by definition less (as in – 0 handbreadths!). Perhaps it depends on the height of the sides before the arch begins.
They agree, however that if its lower section (before the arch begins) was ten handbreadths in height [the doorway] is subject to the obligation.
And Abaye stated: All agree that, if [an arched doorway] was ten handbreadths high but its lower section was less than three [handbreadths in height], or even if the lower section was three [handbreadths high] but its total height was less than ten handbreadths, the doorway is not valid at all.
There was reported architectural disagreement about the home the leader of the Jewish community in Babylon – the Exilarch:
‘If you meet the people of the Exilarch's house’, (R. Shesheth) said to (Rabbah b. Samuel), ‘tell them nothing whatever of the Baraitha about the arched doorway’.
Some things are better left unmentioned!