Filled the grid, found the quotation, but stuck on its interpretation. The title "The square and the compass" is relevant, but can't see beyond freemasonry, which I'm sure is irrelevant. 8 cells have to be changed: 4 are obvious, but where are the other 4?
I'm having the same difficulty as miqq. stymied's advice is welcome, and describing the cells which the quotation suggests in terms of a compass is fairly clear - but do these cells have two letters each? Surely not, as this would be made clear in the preamble. Are the 8 cells whose contents are replaced symmetrically disposed?