I'm making a barrel like cake and want to lay it on it's side (3D look). I know that I will have to cut part of the cake off, to level it out so it will not roll, but do I fill it the same way I would a stacked round cake? Do I have to worry about filling sliding off or will it hold on it's own? Also, do I need dowels going sideways to hold it together? I will be using fondant as my cover. Hope this makes sense. I'd really appreciate the help.

Gravity will win and the layers will separate from one another unless you build it around a support system with "bookends" to keep it together. Otherwise, it needs to be build from the bottom up and carved to the shape.

Please also see:

http://cakecentral.com/cake-decorating-ftopict-724549.html

HTH