If the floor and ceiling (and walls) were leveled and flattened and brought to plumb with a straightedge scraped in with the three-plate method [1] (Popularized by Whitworth in the 1830s, but the ancients made straight edges and flat plates too), then they were actually not 90 degrees at the corners!

[1] https://ericweinhoffer.com/blog/2017/7/30/the-whitworth-thre...