Because it's easier to build from the ground up, we left off the interior floors. Now the first order of business is to construct the stairs from the ground floor to the first floor.
First step: use wooden blocks to make the stairs and first landing (this was a bit like making stairs as a kid from a set of wooden shapes :) I had cut these shapes out downstairs in the workshop ahead of time, and just brought everything upstairs to assemble. Everything is pine.
Second step: glue in the staircase: I'm using a standard, easily-available narrow, steep, open staircase, which I'll stain dark brown. My idea is that the stairs to the first landing (as shown above) are stone, the next flight is wood. I had to cut the top step off, but it fits quite well and takes up very little space. You can see the member I put in to support the first floor, and the notch I put in it for the wall of the stairwell.
Here is the room so far: