Humble wallHumbler walls
The walls in question are humble in the sense that they are lowly and unpretentious in the sense that they belong to humble people and they are low in the sense that they are not as tall as those of the grand palaces and pyramids.
I do however question the use of the comparative 'humbler'. It is not clear to me what they are supposed to be humbler than.
We all remember the pyramids and pharaohs, but advances which were, in the long term, just as significant were being made behind humbler walls.
There are problems with this sentence. Either it
- implies that pharoahs have (or are) walls!
or it
- implies that advances were being made behind the walls of pyramids. That is rather dubious. Pyramids were tombs -- there wasn't much going on behind their walls once said walls had been built. [This claim is challenged, explored further, and I believe justified by me in the comments below]
A quick reading of the sentence yields its meaning. As you say, it suggests that ordinary people were making advances -- not just the great engineers who were responsible for the pharaohs' pyramids and palaces.
I don't agree that the grammar is crazy. It is quite possible to say erroneous or meaningless things and still speak in perfectly good grammar. Instead I would say that the sentence does not stand up to strict linguistic analysis from the point of view of its semantics.
EDIT 13 Oct 2015
I have amended the wording of this answer in response to some of the comments below. I still believe that the sentence in question is infelicitous and my reasons are given here and in the comments.