This was our wedding cake, the idea started as special request from my husband for a Mario teamed cake.
The cake is simply a 12' round, 10' and 6' on top. It is covered in fondant and all the decoration are made out of fondant. There are about 8 dowels in the bottom layer and 6 in the middle to support the weight of the cake and the topper.
I got really lucky and instead of tinting the fondant (which kills my wrists and take a really long time) I found spray can food coloring. One can covered the entire cake (although did need a second one to touch up the color). Be aware when using- you might want to cover the whole kitchen, or spray in an isolated area. My kitchen was blue for about a week after the weeding.
To create the couch for the topper I simply used a block of fondant which hardened and was no longer useful for anything else, but you could also use rice crispy treats. I layered the already tinted fondant on the couch to create the illusion of fabric. As for the characters themselves; my self, my husband and of course Mario, are made out of solid fondant which I had to sculpt to fit the couch- toothpick help to create joints and give support and should be used in large body parts.
The Goombas and Koopas I drew with a toothpick on rolled out fondant and cut out using a hobby exacto knife, this gave me the detail I needed and worked extremely well. Once shape was cut out I used a brush to color the characters.
This cake took a lot of patience but was worth it. We got a wedding cake that no one will forget.