Raycaster Help

Sup /prog/?
I'm writing a raycaster game, and I understand the concept well enough. However, I am confused as to how one would implement multiple floors and heights (such as to get a high up view of the entire map). I know that Doom does things like this. I suppose I could draw every conceivably visible wall from the edges of the map, but that would be inefficient as hell. Is there some special technique?

You're not talking about binary space partitioning are you?
https://en.wikipedia.org/wiki/Binary_space_partitioning

That's how doom decides what and what not to draw.

I'm not sure what you mean when you say multiple floors, Doom is incapable of multiple floors, but the room you are and what you can see is dictated by the BSP algorithm. So rooms can overlap eachother.