Wait... "the surface"? Are they underground? Weren't these walled cities, wouldn't the outside be more appropriate?
Or am I just misunderstanding the use of the word... english isn't my first language.
The cities are basically bunkers where the living quarters/shelters are underground.
So in this context referring to "the Outside" as "the surface" is .....more or less convention in Sci-Fi.
Because "The Outside" can be many things, depending on the setting, but "the surface" makes it clear it's a ( usually supposedly hostile ) planetary surface.
hmmm... a reading recommendation for this trope would be Tanith Lee's "Don't Bite the Sun".
It's one of those SF novels that... would not stand a chance of being published nowadays, doubly so because it's
supposed to be Young Adult.. for 1976....