I used the Wilton “Romantic Castle Cake” set and followed their directions. It included a 10 inch 2-layer cake topped with a 6 inch 2-layer cake (on a cake board).
I iced the Princess Castle Cake all in white buttercream and added almost 300 fondant flowers. I placed the entire cake on a large board covered with cake foil and added the plastic Disney Princess figurines. Cinderella even had her horse and carriage on the side.
The fondant flowers should be made several days or weeks ahead of time and stored in an airtight container.
You should also do the castle pieces ahead of time. They store well and should not be covered so they will harden before placing them on the cake. The instructions in the set tell you to ice the pointed pieces for the tops with buttercream and add the cake sparkles. The windows have fondant flowers and leaves along the bottom. Doing this ahead of time will allow them to harden and stay in place.
The cake took several hours to complete. You should do everything you can ahead of time but you will have to wait until that day to fully assemble the cake and add all of the decorations. The leaves around the flowers were especially time consuming.