The maximum riser height and minimum tread width in the building code will determine how thick the landing can be. Your plan has about half the risers needed.

I thought in the northern hemisphere the stair has to descend clockwise?

Not that I've ever read or been told. I am right handed and would much rather descend with the right hand. It will pass code as per the city. I wanted it to be positioned exactly as shown.

I have no knowledge about building of spiral staircases, but I do know something about their history, and that might interest you.

Mike mentioned that being right-handed makes a difference in the "feel" of walking up a spiral staircase -- he's on the right track. Back in the middle ages, castle owners designed spiral staircases to benefit a right-handed swordsman. The stairs'd be built so that a swordsman could stand on the wider portion of the step and have full use of his right arm. In contrast, his opponent would be forced to fight while standing on the narrow end of the step, and use of his right arm would be impeded by the staircase itself. His chances of winning his way up the steps was greatly impeded by the design.

Wanna know about murder holes? Arrow loops? Crenelated battlements? Parapets? One of my degrees is in history.

In more practical terms, do you have any experience with spiral staircases? I thought they were super cool until I stayed in a beach house in which the second floor was ONLY accessed by a single spiral stair. Getting a suitcase up those stairs was a pain. I wouldn't want to move a bed upstairs or carry laundry on a regular basis. I think a spiral staircase is cool as a second staircase ... but not as an only staircase.

Military history teaches that castles were taken by entering over the top of the embattlements using rolling towers and ladders after long sieges. So, it seems more likely that the enemy would be coming down small isolated spiral stairs in the castle walls. And it seems equally likely that neither side would chose to fight in a narrow stairway.

When a defensive military position is lost, falling back to a predetermined secondary position is preferable to separating forces and fighting one on one in isolated spaces. However, the defenders might have set a fire in the bottom of the stair before falling back; they liked fire a lot.

Surprisingly, I found that getting smaller furniture up was a breeze. I used straps on the furniture and hoisted it up along the outside of the railing. I think the items "felt" less heavy and bulky due to the leverage and length of the straps as opposed to carrying items at chest or waist level.