When I made turrets for a castle cake, I wrapped the fondant around paper towel tubes that I had rubbed with shortening for the fondant to stick to. I made them about 5 days ahead of time, and the fondant did harden.
Not sure if my reply comes to you in time but I do have a video (slide show version) on you tube on making the castle cake turrets as well as the a video version on my website. My website videos are for members only and it’s free.
I make the turrets with gum paste and roll them around card tubes. The pictures here amongst my photos has one one of my castle cakes I made.
All the best.