I was thinking of a way to do ramps and stairways. After a lot of thought I found a way, and, in the process of adding it, I also made it separate the walls and floors properly, and the walls are now more upright.
My next task will be speeding up the renderer. It is going at about 5-7 FPS at 800x600 at the moment, but I think it can go much faster.
I might also experiment with adding ramps.