I love the extra tall tiers, but exactly how are they constructed?
I recently made an 8" tall 8 inch square cake. A baby block, so it had to be evenly tall/wide/long...whatever. I baked 4 layers, filled and stacked like it was two separate cakes, and stacked those together. They were iced and smoothed separately, taking great care to ensure once they were stacked, they aligned perfectly, and there wasn't a gap where the two cakes met. I had to fill and smooth a teeny bit once it was assembled, and once the fondant went on, you couldn't tell it was two different cakes.
Here's a great photo tutorial: http://www.flickr.com/photos/deliciously_decadent/sets/72157621850372685/
I would do it like Jamie