Here is my solution. A lot of SNOT, hinges, two jumpers and tile inside the structure to make it very solid.
Octagon is not 100% correct in shape at the base: 1/2 stud short in one direction and 1/4 stud offset in oposite. Quite a chalange to made such shape solid and fit slope bricks. Three 1x4 plates between 2nd and 3rd line of slopes solve the problem.
@Medieval Guy: I was inspired by your roof and wanted to fix hollow top and loose slopes.
@Aiden Bryant & Eric W.: Tower for the roof is allready made, but it is a part of a large project not finished jet. I need couple of months more...be patiend.
I wonder why they do not have this technique in a few of the Harry Potter sets, Hmmmm...
This would look really cool on something though. Thank you for sharing your technique and how you did it. I admire you for that! A lot of good builder's do not take the time to explain how they do their special techniqes.