They're built out of large slabs of concrete with steel beams in between and have flat roofs - that's about it. Terrible to live in, but neither a car at speed

They're built out of large slabs of concrete with steel beams in between and have flat roofs - that's about it. Terrible to live in, but neither a car at speed nor a falling tree can punch a hole in them. Also they weren't built as close to each other as modern buildings are, so debris from one mostly don't reach the other.