Concerning cathedrals
One thing that is rarely mentioned and a suprize for myself is the fact that in the process of building the cathedral there were passages built in the larger structures to facilitate, inspection and repairs.
The reason that little mention of these features is that, in the passing of years; debris closing the passages, ignorance and general lack of interest, allowed this interesting titbit to be forgotten. (Except if you are a hunchback. "The bells, whoop, whoop, Nyananana, tee hee *jump* *clasp rope* *
DONG!!!! *)
Now, I am remembering off the top of my head, as the book I had is long gone or packed away.
The passages were very narrow and in the major supporting structures of the cathedral, often coming out on a ledge serving as an staging for one of the other structural features both inside and outside.
This allowed for masons and others to inspect during the building process and to help maintaining afterwards without the time, material, and additional cost of building a suitable means of acheiving the same goal. (Not to mention grumpy Bishops..."Twenty eight years to build this place and now you have to build scaffolding to look for a leak. I should have you excommunicated...")
The remarkable achievment is made more so through the discovery of actually how well thought out, developed, and organized the making of the wonderful examples of building we see today.
And thus allowing a DM's plot device where a character (good, evil, NPC and more...Bruuuhahahaha) could move about in a building, using forgotten passages and virtually be undetected.
"Invisibility... We don needs no stinkin invisibility."