Trouble with Armor and Worldbuilding

[Bruno]

Getting the idea gothic arch relies on either a fairly good understanding of the distribution of forces, or a lot of trial and error. An extra thousand-or-so years is a lot of extra time for someone to stumble across a good idea.

In part, the cathedrals (among other big things) were built at the very edge of technical capabilities at the time, and sometimes just sliiiiiightly beyond the local capabilities. There were certainly failures, and builders would have to clean up a collapsed wall and try again, hopefully not making the same mistake.