If you are going to put a double course of blocks for the entire height of the wall, the floors shouldn't be too hard.
Use inch square dowels to make a beam basis for your floor. The dowels go into the wall, which you used custom-cut blocks to anchor.
For four-sided rooms (square, rectangle, even trapezoids if you have them.):
If you want to use 1/4 inch x 1/4 inch, space them about 1 inch apart. For 1/8 inch X 1/8 inch, set them about 1/2 inch apart. If you are still going to build the whole castle so that you can remove each floor, you want to set the "beams" a MINIMUM of three vertical courses above the split point. At the level of the beams, using a Dremel or router, cut a grove into the blocks to accommodate a 1/6 inch thick piece of wood, cut to fit the room exactly (use multiple pieces if needed), plus half the width of the block. This is the base for the flooring material (tiles or those skinny sticks).
For Round or more than four sided rooms or triangles:
Option 1, the Simple way: Space the "beams" in a wheel pattern under your floor base, using a smaller piece of wood in the same shape of the floor in the center (where they join) to secure them.
Option 2, the Complicated Way (but it looks better): Divide the shape into triangles as equal as possible., place beams along these lines, with the ends going toward wall into the wall. Then, divide these triangles into equal distances, and place beams along these lines into the wall. Use the small shape like above as an anchor for the center.
Option 3, the "What the
was I thinking, this is a pain in the ass. But DAMN that looks so bad ass that even Satan would cry!" Method (this works for any shape other than rounds):
Divide the floor into equal sized triangles as above (each triangle has to be independent of the other, i.e. the sides can touch, but not share (so the sides going into the walls are always doubled). Add a beam to the center of each of the main triangles (dividing them in half), by trimming a beam to a point, and notching the original beam group in the center to half depth, doing the same with center point beams. Divide the center beams into equal distances, and place a beam at the distances (cross-bars), notching each beam to half depth ALL the beams are at equal depth. (You should have something like a spider-web now.) Between each cross-bar, place a small beam at an angle. Make all the angled beams (supports) angle towards the walls, making each triangle have smaller triangles all with the fat end going to the wall. (This should make each ring look like it's kinda like a circular saw blade). Use a small piece of wood the same shape as the floor to anchor the wood as above.
If you would like, I can see about getting some sort of blueprint type thing together eventually, i.e. you should keep working on the ground floor until I can get them to you....