I did one (it is in my photos) a while back.
- I used gum paste, rolled it moderately thin, put it in the freezer for a few minutes (this makes it easier to cut/avoid pulling), then cut using a card stock template and exacto knife. I had found the template to print somewhere on the internet.
- I made 3 sections for each side of the tower (so a total of 12 pieces, 4 each for the top, middle and bottom of the tower). I also made an extra of each section just in case.
- Then cut 2 square pieces with centers removed to use as the platforms in between the sections (this helps with assembly). So a square, with a smaller square cut out of the center, 1 sized to fit between the bottom and middle of the tower, and 1 square to fit between the middle and top sections.
- I made a model of the tower out of cardstock. This model was my form to let the gum paste pieces dry overnight at the correct angle.
Once all the pieces were dry, I assembled each section separately (4 pieces to form top, 4 to form center, 4 for bottom) with gum glue. Let that dry, then built the tower with the square platforms to hold together each section.
The most tedious part, was the iron work. This was done with my clay gun and black fondant (or gum paste), attaching each piece with a small amount of gum glue. You could pipe it with Royal Icing if you prefer.