“Wouldn’t there be a huge problem if the ground floor is 18 inches below street level?”
Not in Chicago.
Some areas of Chiraq had several FEET of fill added, making the front window, appear to be an area well.
See the third photo down at link:
https://www.wbez.org/shows/wbez-blogs/south-chicago-past-and-present/59510e5d-09e1-4c1a-959f-540c925d5122
That entry door seems level, or even slightly above, the adjacent sidewalk. Was it built that way, or was the door raised and that little bridge added later? In which case you’d need steps inside the doorway in order to be standing on the first floor.
Same in the earliest parts of Sacramento, which date to the 1850s and is now a state park. Because of frequent floods from the Sacramento River, in the 1860s they brought in fill to raise the grade. The basement in a number of the historic structures is actually the old first floor