There are arches for the main support including the center column. It branches out and then back in to hold the domes that support the spiral. I used small rotating flat pieces on two corners of each unit giving it the spiral effect.
Oh my god this is high! I tried on day build such a tower, on a 2x2 base. I reached 1meter than it broke together. On your creation, I'm just wondering how this all is not falling of! So, very great job! 5/5 ( ps: nice porsche poster on the second picture) :-)