The first step is making your Grey Fondant. (I linked to my post on it because it can be a PAIN!) I make sure my cake is nice and chilled, then roll the fondant out with one of these mats-
It works really easily- making sure the cake is cold helps the fondant to set quickly as you lay it over the sides. To make the side turrets I roll a long piece of Rice Crispy treat on a mat to get it as smooth as possible, spread a thin coat of frosting over it and then cover it with a thin layer of white fondant, then the same rolled grey fondant. I spray the whole thing with Duffs edible silver spray. For the tops I used sugar cones covered in fondant- you can get as simple or complicated as you like with those! Here is how I did the Princess Topper.