Ah, the spiral staircase. Had to build one for the SGC, because the conference room in the show has one (off camera) that leads directly to operations.
All you need to do to build a spiral staircase is:
1) decide what type of stairs to use(stone, wood, metal grating, etc..)
2) decide what type of material(foamboard, plywood, plastic,etc.)
3) decide how many turns it shall make, taking into account and respect for
the space need and the height. Remember, it must be tall enough between each spiral to accomodate a figure from head to toe with at least a 1/2" allowance.
4) decide how round/large the center beam/pole is. This is the most important part, as a larger circumference on the pole means shorter steps in width.
5) decide if it is really worth it (I'll let you know later
) When making the stairs themselves, you'll need to draw out at least 4 to five circles of equal circumference. Then, using a ruler, bisect the circles at least thrice which should yield at least six steps of equal size.
Take your center pole, and trace around the edge of it after centering it in the middle of each drawn and bisected circle. When done, now you have a good idea how small and wide your steps are.
Cut out all the steps, and depending on your material choice, starting from the bottom, glue the inner end of the steps to the pole, allowing the requisite space for each 1:18 scale or 1:24 scale footstep. Remember to keep the height allowance of the figures used in mind when you complete one full spiral.
On your foundation and subsequent upper floor, use one of the uncut circles to provide a template to cut the hole in the roof, where the spiral staircase will appear through, Leave enough spiral turns to have to figure be able to walk from the last upper step, to the floor of the above room.
When done, decide if you want a guard rail. Most spiral staircases have them, but the material you use must be flat, pliable, and attached to the steps from the outer center of each step. Me? I'd say bugger the guard rail. Too much work and a real &%$@# to get to stay put.
When complete, treat yourself to a movie and meal. You'll need it from the aggravation of all the measuring and trying to get the spiral the right height for the figures.