Built of Poured Concrete
The house we live in was built of poured concrete. I figure the "older" part was built some time in the late teens or early twenties of the last century. The wiring was all two-line, run in conduit.
The "newer" part was added on in the mid-1930s in an entirely different style, heavy-duty Romex throughout.
It was easy to pull and replace the conduit with new three-line wiring. The Romex... still trying to figure out what to do about that.
_/\_
Favorite rewiring story: Chicago apartment building, built around 1910, remodeling in the mid-1980s. We removed a light fixture in the ceiling and... smelled gas. They had run the wiring through the old gas lighting pipes! Just a residual whiff, not connected to gas anymore, but kinda hair-raising for a moment.