An astute observation. Many Hive-spires were initially built around mountains, though some were just built on the Detritus of other buildings.
[Gaunt favors her with a brief, but genuine smile.]
It's just a spire part of the city, the whole conurbation is a much larger structure. Hives can cover almost an entire continental plate, depending on the age of the city in question.
[Gaunt follows up regarding Mid-Childa after a brief pause.] So you also have open spaces? Agri-combines and the like on your world?