The tavern conversation got me thinking about this. From the outside the tavern looks extremely small in comparison to its inside. Having this for the tower would nice since if the floors were as long and wide in ratio to the tower structure's wideness and length, each floor would be pretty small. Having each floor as big as the castle's town square (area inside the walls of the second leystone/trade center) would be a good size. The floors could get smaller the higher up you go but the bottom floor would have to start off huge, otherwise the top floor would be tiny. The top floor should be at least big enough for ranged classes to fight while not being affected by aoe. Assuming the big bad boss (hopefully Crom) is up there.
In regard to the spiral staircase problem. Maybe have a curved staircase against the wall going all the way up the tower to the top floor. Have a long step section at each interval where the staircase and the next floor meet. For the problem in which no one could stand under the staircase, either have a pretty looking material as a fill in or add in a good ol' CH barrier.
Some thing like that. The floors would make it so that the walls/material fill in of the staircase wouldn't affect the floor below it. If the staircase in fact has fill in/walls and not a barrier. This would sustain the pretty spiral design concept we all love so much, and give less work to the OTM team since the staircase would keep a consistent, simple and professional look and be the same thing everywhere.
I had a picture but the picture is too big, here is a url. Its the second picture on the page.
http://blog.apicius.it/?p=601
I hope it isn't against the forum rules.