Do a taste test of the both to make sure you like it/them. For tall cakes I use packing crates if they're stacked or put each tier in a cakebox and assemble on site. But notice how on t.v shows they hardly ever use boxes ?
Check out http://cakecentral.com/gallery/34174/christening-cake. I am thinking a two-tier cake, maybe a 16" and a 12", with the 12'' being a replica of the topper here. You should be able to work in quite a few decorating genres. You can do a baby (F.I No.5 is good) and an angel if you're unsure about crosses and chalices.
The height of the cake would determine the process used for me. But for something like that I would do like Bakencake said but I'd do the base first and then the dome. What I did a couple times was measure EXACTLY and then thin/bevel the edge so thet when it came time for the seam, it was barely noticeable.
You SIMPLY MUST wait till the cake is thawed before applying fondant. That it's already iced is a huge bonus. Applying fondant to cold cake causes condensation and you get a sticky, icky possibly runny fondant.
Isomalt for sure. Do you have a SASSIE? Having one makes life with isomalt way easier, but if you don't just pour the boiled isomalt into a glass cup with a pouring lip and fill up your molds. There's a recipe for isomalt gems on [url]sugarcraft.com.[/url]
Stack round cakes to the height you want for your glass. Wrap the 'glass' in fondant. Stick a long dowel through it to ensure it doesn't go anywhere and to be the 'pour'. Use isomalt, candy melts, gumpaste or chocolate for the 'pour'. Glue the can to the end of the dowel. The ice cubes are isomalt .
Cover a length of PVC in silicone paper. Measure the circumfrence of the PVC. Cut GUMPASTE to the height you want and the circumfrence just measured. Emboss and wrap (not too tightly) around the PVC. Let dry. Your crenelations can be made using The straight frill cuter from FMM (No 9-12) Icecream cones covered in chocolate, candy melts or icing can be left plain or dipped in sprinkles. You can turn a cone upside down and crenelate it for where the princess is standing.