Some important tips about topsy turvy cakes:
Use a very dense cake. If you use mixes, use onlly half of the oil (or less) that it calls for.
Make each cake tall, if you don't it will look 'squat'. Use 3 two-inch layers for each tier.
Use a very thick double dam because when you carve the cake you will loose some of the dam.
As someone else said, make sure that the carved out hole is perfectly level (put a cake pan inside with a level on top to check).
Make sure to use dowels (but only within the hole obviously) and a sharpened wooden dowel down the middle.
I've made two topsy turvy cakes (you can see them in my photos). I almost threw up the day I made the first one, I was so worried because the lady I made it for had to travel for a good hour with it in the car. I heard it arrived safety and was the talk of the party, then I could breath again! I owe everything to Sharon Zambito. She's awesome and her DVDs are awesome! Good luck! Any questions, just ask!