For now it seems to me you could just build some crazy relief and The Wall to render your city unreachable?
Maybe after land form change there should be some blocking check (is it pissible to reach the buildings?) and the message if the relief is unpassable, if player does not fix it - the relief is getting planar. The people need to get out of their houses somehow, right?
There are often the case for me with buildings getting underground on repair.
Maybe the land should be more "crumbly"? for example, if there is some crater left by scrap falling from the sky, it should be filled by adjacent land material with some blocks being destroyed or just thrown sideways with impact. Unnatural columns will crumble, unnaturally deep valleys will be filled.
Then maybe landships could pass some hard terrain by flattening/redistributing it with their tracks?
And all heavy ships shold probably crumble the trees without slowing down much?